V4L/DVB (7613): em28xx: rename registers

Now, all registers will follow the same convension:

EM28XX_R<reg_number>_<reg_name>

This allows to associate a register with its value, and also with a canonical
name. Also, registers that are specific to a given chip were renamed accordingly,
as EM2800_foo (for 2800 only registers) or EM2880_foo (for registers that started
to appear on em2880).

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>