media: staging: atomisp: fix string comparation logic

it makes no sense to use strncmp() with a size with is
bigger than the string we're comparing with.

Fix those warnings:

    drivers/staging/media/atomisp/pci/atomisp2/atomisp_fops.c:776 atomisp_open() error: strncmp() '"ATOMISP ISP ACC"' too small (16 vs 32)
    drivers/staging/media/atomisp/pci/atomisp2/atomisp_fops.c:913 atomisp_release() error: strncmp() '"ATOMISP ISP ACC"' too small (16 vs 32)
    drivers/staging/media/atomisp/pci/atomisp2/atomisp_ioctl.c:2751 atomisp_vidioc_default() error: strncmp() '"ATOMISP ISP ACC"' too small (16 vs 32)

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>