[media] em28xx: fix and separate the board hints for sensor devices

The current board hint code is mixed together with the sensor detection and
initialization code. It actually selects a board depending on the detected
sensor type only, with the result that 3 of the 6 webcam boards are currently
dead.
Separate it and move it to em28xx_hint_board() which already contains the board
hints for analog capturing+TV and DVB devices.
This way, we have all board hints at a common place which makes it easier
to extend the code and reduces the risk of regressions.
It also makes it possible again to use the boards EM2750_BOARD_DLCW_130,
EM2820_BOARD_VIDEOLOGY_20K14XUSB and EM2860_BOARD_NETGMBH_CAM (using the module
parameter "card").
NOTE: the current board hint logic for webcams is preserved. Not more not less.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>