V4L/DVB (9747): em28xx: Properly handles XCLK and I2C speed

The previous patches removed XCLK and I2C magic. Now, we finally know
what those registers do. Also, only a very few cards need different
setups for those.

Instead of keeping the setups for those values inside the per-device
hack magic switch, move the uncommon values to the board-specific
struct, and have a common setup for all other boards.

So, almost 100 lines of hacking magic were removed.

A co-lateral effect of this patch is that it also fixes a bug at em28xx-core, where xclk
were set, without taking any care about not overriding a previous xclk setup.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>