V4L/DVB (12410): em28xx: Move the non-board dependent part to be outside em28xx_pre_card_setup()

em28xx_pre_card_setup() is meant to contain board-specific initialization. Also,
as autodetection sometimes occur only after having i2c bus enabled, this
function may need to be called later.

Moving those setups to happen outside the function avoids calling it twice without
need and without duplicating output lines at dmesg.

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