ice: fix return value check in ice_gnss.c

kthread_create_worker() and tty_alloc_driver() return ERR_PTR()
and never return NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Fixes: 43113ff7

 ("ice: add TTY for GNSS module for E810T device")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>