spmi: Add a check for remove callback when removing a SPMI driver

When removing a SPMI driver, there can be a crash due to NULL pointer
dereference if it does not have a remove callback defined. This is
one such call trace observed when removing the QCOM SPMI PMIC driver:

 dump_backtrace.cfi_jt+0x0/0x8
 dump_stack_lvl+0xd8/0x16c
 panic+0x188/0x498
 __cfi_slowpath+0x0/0x214
 __cfi_slowpath+0x1dc/0x214
 spmi_drv_remove+0x16c/0x1e0
 device_release_driver_internal+0x468/0x79c
 driver_detach+0x11c/0x1a0
 bus_remove_driver+0xc4/0x124
 driver_unregister+0x58/0x84
 cleanup_module+0x1c/0xc24 [qcom_spmi_pmic]
 __do_sys_delete_module+0x3ec/0x53c
 __arm64_sys_delete_module+0x18/0x28
 el0_svc_common+0xdc/0x294
 el0_svc+0x38/0x9c
 el0_sync_handler+0x8c/0xf0
 el0_sync+0x1b4/0x1c0

If a driver has all its resources allocated through devm_() APIs and
does not need any other explicit cleanup, it would not require a
remove callback to be defined. Hence, add a check for remove callback
presence before calling it when removing a SPMI driver.

Link: https://lore.kernel.org/r/1671601032-18397-2-git-send-email-quic_jprakash@quicinc.com
Fixes: 6f00f8c8 ("mfd: qcom-spmi-pmic: Use devm_of_platform_populate()")
Fixes: 5a86bf34 ("spmi: Linux driver framework for SPMI")
Signed-off-by: Jishnu Prakash <quic_jprakash@quicinc.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20230413223834.4084793-7-sboyd@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/spmi/spmi.c

@@ -350,7 +350,8 @@ static void spmi_drv_remove(struct device *dev)
 	const struct spmi_driver *sdrv = to_spmi_driver(dev->driver);
 
 	pm_runtime_get_sync(dev);
-	sdrv->remove(to_spmi_device(dev));
+	if (sdrv->remove)
+		sdrv->remove(to_spmi_device(dev));
 	pm_runtime_put_noidle(dev);
 
 	pm_runtime_disable(dev);