V4L/DVB (9658): em28xx: use em28xx_write_reg() for i2c clock setup

Convert the calls that write the i2c clock register over to the new
em28xx_write_reg() function that allows for or'ing bits

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