tpm: Fix null pointer dereference on chip register error path

If clk_enable is not defined and chip initialization
is canceled code hits null dereference.

Easily reproducible with vTPM init fail:
  swtpm chardev --tpmstate dir=nonexistent_dir --tpm2 --vtpm-proxy

BUG: kernel NULL pointer dereference, address: 00000000
...
Call Trace:
 tpm_chip_start+0x9d/0xa0 [tpm]
 tpm_chip_register+0x10/0x1a0 [tpm]
 vtpm_proxy_work+0x11/0x30 [tpm_vtpm_proxy]
 process_one_work+0x214/0x5a0
 worker_thread+0x134/0x3e0
 ? process_one_work+0x5a0/0x5a0
 kthread+0xd4/0x100
 ? process_one_work+0x5a0/0x5a0
 ? kthread_park+0x90/0x90
 ret_from_fork+0x19/0x24

Fixes: 719b7d81 ("tpm: introduce tpm_chip_start() and tpm_chip_stop()")
Cc: stable@vger.kernel.org # v5.1+
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

drivers/char/tpm/tpm-chip.c

@@ -77,6 +77,18 @@ static int tpm_go_idle(struct tpm_chip *chip)
 	return chip->ops->go_idle(chip);
 }
 
+static void tpm_clk_enable(struct tpm_chip *chip)
+{
+	if (chip->ops->clk_enable)
+		chip->ops->clk_enable(chip, true);
+}
+
+static void tpm_clk_disable(struct tpm_chip *chip)
+{
+	if (chip->ops->clk_enable)
+		chip->ops->clk_enable(chip, false);
+}
+
 /**
  * tpm_chip_start() - power on the TPM
  * @chip:	a TPM chip to use
@@ -89,13 +101,12 @@ int tpm_chip_start(struct tpm_chip *chip)
 {
 	int ret;
 
-	if (chip->ops->clk_enable)
-		chip->ops->clk_enable(chip, true);
+	tpm_clk_enable(chip);
 
 	if (chip->locality == -1) {
 		ret = tpm_request_locality(chip);
 		if (ret) {
-			chip->ops->clk_enable(chip, false);
+			tpm_clk_disable(chip);
 			return ret;
 		}
 	}
@@ -103,8 +114,7 @@ int tpm_chip_start(struct tpm_chip *chip)
 	ret = tpm_cmd_ready(chip);
 	if (ret) {
 		tpm_relinquish_locality(chip);
-		if (chip->ops->clk_enable)
-			chip->ops->clk_enable(chip, false);
+		tpm_clk_disable(chip);
 		return ret;
 	}
 
@@ -124,8 +134,7 @@ void tpm_chip_stop(struct tpm_chip *chip)
 {
 	tpm_go_idle(chip);
 	tpm_relinquish_locality(chip);
-	if (chip->ops->clk_enable)
-		chip->ops->clk_enable(chip, false);
+	tpm_clk_disable(chip);
 }
 EXPORT_SYMBOL_GPL(tpm_chip_stop);