media: atomisp: atomisp_gmin_platform: check before use

solve this smatch warning:
	drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c:842 gmin_v1p8_ctrl() warn: variable dereferenced before check 'gs' (see line 832)

By moving the check to happen before its usage.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>