staging: rtl8188eu: replace (a == NULL) to (!a) in rtw_drv_init

It is a checkpatch cleanups:
CHECK: Comparasion to NULL could be written ...

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>