scsi_command.py: fix decode_all_bits, loop until bit 0 is NOT set

We shift all bitmasks until the least significant bit we have masked is 1
not until it is 0.
We do these shifts so that any fields we mask out will be numbered sanely.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #18 from sahlberg/decode-bits-bug

Decode bits bug