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>