cp2112_i2c_xfer() only supports a single i2c_msg.  More than
one message at a time just returns EIO.  This breaks certain
important cases.  For example, the at24 eeprom driver generates
paired write and read messages (for eeprom address and data).

Since the device doesn't support i2c repeated starts in general,
but does support a single write-repeated-start-read pair
(since hardware rev 1), we recognize the latter case and
implement only that.

Signed-off-by: Ellen Wang <ellen@cumulusnetworks.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>