HID: cp2112: add I2C mode

cp2112 supports single I2C read/write transactions.  It can't combine I2C
transactions.

Add master_xfer, using similar code flow as for smbus_xfer.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>