flash/nor/str9xpec: fix help and usage for command "part_id"

Remove the numeric parameter <bank> from help and add it to usage

Change-Id: I630ca5450955210299f54e16fe5c9e8e51b3b266
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5022


Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins