Some minor cleanups.  First, some locking in the bus-reset.  Next, we move
current_sg into struct us_data (why make more memory allocation issues for
ourselves?).  Next, we change sm_state into a normal variable, since it
shouldn't require atomic_t anytmore.  Finally, we remove some references to
a couple of flags that don't do anything anymore.

# Fix device locking during the bus-reset routine.
#
# Embed current_sg in struct us_data.
#
# Make us->sm_state a regular int instead of an atomic_t.
#
# Remove a couple of references to the START_STOP and IGNORE_SER
# flag bits.