i2c: octeon: Prevent error message on bus error

The error message:

[Fri Feb 16 13:42:13 2018] i2c-thunderx 0000:01:09.4: unhandled state: 0

is mis-leading as state 0 (bus error) is not an unknown state.

Return -EIO as before but avoid printing the message. Also rename
STAT_ERROR to STATE_BUS_ERROR.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>