ice: Don't dereference NULL in ice_gnss_read error path

If pf is NULL in ice_gnss_read() then it will be dereferenced
in the error path by a call to dev_dbg(ice_pf_to_dev(pf), ...).

Avoid this by simply returning in this case.
If logging is desired an alternate approach might be to
use pr_err() before returning.

Flagged by Smatch as:

  .../ice_gnss.c:196 ice_gnss_read() error: we previously assumed 'pf' could be null (see line 131)

Fixes: 43113ff7 ("ice: add TTY for GNSS module for E810T device")
Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_gnss.c

@@ -96,12 +96,7 @@ static void ice_gnss_read(struct kthread_work *work)
 	int err = 0;
 
 	pf = gnss->back;
-	if (!pf) {
-		err = -EFAULT;
-		goto exit;
-	}
-
-	if (!test_bit(ICE_FLAG_GNSS, pf->flags))
+	if (!pf || !test_bit(ICE_FLAG_GNSS, pf->flags))
 		return;
 
 	hw = &pf->hw;
@@ -159,7 +154,6 @@ static void ice_gnss_read(struct kthread_work *work)
 	free_page((unsigned long)buf);
 requeue:
 	kthread_queue_delayed_work(gnss->kworker, &gnss->read_work, delay);
-exit:
 	if (err)
 		dev_dbg(ice_pf_to_dev(pf), "GNSS failed to read err=%d\n", err);
 }