The ice_allocate_sf() function returns error pointers on error.  It
doesn't return NULL.  Update the check to match.

Fixes: 177ef7f1 ("ice: base subfunction aux driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/6951d217-ac06-4482-a35d-15d757fd90a3@stanley.mountainSigned-off-by: Jakub Kicinski <kuba@kernel.org>