V4L/DVB (9744): em28xx: cleanup XCLK register usage

Convert over to setting the XCLK register usage with the new em28xx_write_reg()
function.

Thanks to Ray Lu from Empia for providing the em2860/2880 datasheet.

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