i2c-algo-bit: Let user test buses without failing

Always failing to register I2C buses when the line testing fails is a
little harsh. While such a failure is definitely a bug in the driver
that exposes the affected I2C bus, things may still work fine if the
missing initialization steps are done later, before the I2C bus is
used. So it seems a better debugging tool to just report the test
failure by default. I introduce bit_test=2 if anyone really misses the
original behavior of bit_test=1.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>