V4L/DVB (13456): s2250: Change module structure

The s2250-board i2c module was converted to use v4l2-i2c-drv.h in
preparation for its subdev conversion.  This change prevented the
s2250-loader from being initialized within the same module due to
the module_init and module_exit function definitions in v4l2-i2c-drv.h.
Therefore, s2250-loader is now its own module, and the header for
exporting s2250-loader functions is no longer needed.

The s2250 i2c module name was "2220-board" in some places, and was
changed to "s2250".

Signed-off-by: Pete Eberlein <pete@sensoray.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>