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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>