[media] e4000: convert to Regmap API

That comes possible after driver was converted to kernel I2C model
(I2C binding & proper I2C client with no gate control hack). All
nasty low level I2C routines are now covered by regmap.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>