[media] em28xx: Move em28xx_register_i2c_ir() to em28xx-input.c

This function is only used in em28xx-input.c so it
makes no sense to have it anywhere but in em28xx-input.c.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>