stellaris: probe() cleanups

stellaris: probe() cleanups

Fix potential memory leak:  make sure the per-bank data
structures are only allocated in probe(), and that calling
probe() multiple times is a NOP.  Use it for auto_probe().

Require probe() to have done its thing:  don't make access
routines cope with it not having been called.  Shrink a
bunch of failure paths; and in some cases, correct them.

Don't needlessly insist on a halted target for probe().

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>