stellaris: avoid chip writes

stellaris: avoid chip writes

Previously "reading" clock info (and part info) also, as a side
effect, wrote the flash timing register.  Instead, be more safe:
"reading" should only read.  Write paths still refresh timing,
coping with changes the application code may have made.

Also rename the routine which sets flash timing, indicating what
it's really doing; it's got nothing to do with a "mode".

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