Split out the i2c related code from the ngene-core.c into a new file called
ngene-i2c.c.  This is part of a larger refactoring of the driver in preparation
for adding analog support.

Note that this patch makes *no* functional change - all I did was cut/paste the
code and add the function prototypes to ngene.h as needed.
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>