NOR: trim range in flash_driver_protect()

NOR: trim range in flash_driver_protect()

When the beginning or end of the specified range of sectors
already has the requested protection status, don't ask the
flash driver to change those sectors.

This will among other things turn command sequences like
this into the NOPs one would expect:

	flash protect_check 0
	flash info 0
		... reports everything as unprotected ...
	flash protect 0 0 1 off

That speeds things up (by whatever work was just avoided).

Also, with Stellaris (which can't unprotect flash at  page level)
this can eliminate some undesirable/false error reports.  (And
finishes fixing a bug currently listed in our bug database...)

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