[media] cx25840: Remove a now unused variable

drivers/media/video/cx25840/cx25840-core.c: In function ‘cx25840_s_frequency’:
drivers/media/video/cx25840/cx25840-core.c:1563: warning: unused variable ‘state’

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