media: s5p-mfc: Add checking to s5p_mfc_probe().

If of_device_get_match_data() return NULL,
then null pointer dereference occurs in  s5p_mfc_init_pm().
The patch adds checking if dev->variant is NULL.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Nadezda Lutovinova <lutovinova@ispras.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>