V4L/DVB (9657): em28xx: add a functio to write on a single register

Introduce a new function that writes to a single register.  This is
useful because the vast majority of register writes are a single
register, and this format permits or'ing register value bits together.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>