flash/nor/stm32l4x: fix minor errors in flash write/async algo

flash/nor/stm32l4x: fix minor errors in flash write/async algo

Fix comment of tested errors in asm src.

List all relevant errors in FLASH_ERROR mask: FLASH_PROGERR was missing
and any trial to re-program already programmed double word ended up
in the error bit held uncleared and flash write permanetly repeating
the error message until reset.

Lock the bank also after unsuccesfull write_block run.

Set async target algo block size to size of double word.

Remove warning in case of write_block success. In case of error
use LOG_ERROR instead of warning.

Change-Id: Ibf6d5e306a4c2eaa43de67d636b4902c737f02f3
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5360
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>