viafb: fix i2c_transfer error handling

i2c_transfer returns negative errno on error and number of messages
processed on success. Just returning this value would give a poor
interface as it is not obvious that you must compare with 2 after reading
1 or n bytes and with 1 after writing 1 byte to determine if it was
successful. To avoid this error prone interface convert the error code
of a successful read/write to zero and all other non-negative values to
an negative error code.
This fixes a regression introduced by
	via: Rationalize vt1636 detection
which resulted in no longer detecting a VT1636 chip and therefore has
broken the output in configurations which contain this chip.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: stable@kernel.org