flash/stm32l4x: use COMMAND_PARSE_NUMBER in command handlers

flash/stm32l4x: use COMMAND_PARSE_NUMBER in command handlers

the usage of COMMAND_PARSE_NUMBER is safer in COMMAND_HANDLERs since it provides
better error checking than strto** functions.

Change-Id: I7e113b06b74f2d8d9cc4c0ce1957994a1c49c964
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6419
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins