flash/nor: fix doc/help and range test for flash protect

flash/nor: fix doc/help and range test for flash protect

Commit 77a1c01ccbb1150ffe749a7373cf6c4dc15ecad0 introduced infrastructure
for utilizing protection blocks of different size than erase sector.
Parts of doc/help kept reading 'sector' instead of 'protection block'.
flash_driver_protect() parameter range testing did not switched
to bank->num_prot_blocks.
This change fixes it.

Change-Id: Iec301761190a1a1bcc4cb005a519b9e5e4fede51
Reported-by: Mark Odell <mark@odell.ws>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3917
Tested-by: jenkins
Reviewed-by: Mark Odell <mrfirmware@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>