py-smbus: Fix i2c_smbus_* error propagation

The Python bindings haven't been updated after commit
330bba29f3d02432e2dca6f85082763b248887ff ("libi2c: Properly
propagate real error codes on read errors"). This led to erronenous
behavior every time an error other than -1 is returned by i2c_smbus_*
functions.

[JD: Edited description.]

Signed-off-by: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>
2 files changed