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: NAntonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>