flash/nor/stm32l4x: cast wrpxxr_mask to uint16_to to print

flash/nor/stm32l4x: cast wrpxxr_mask to uint16_to to print

Fix build error on Mac OS X Catalina (10.15.4) caused by formatting
stm32l4_info->wrpxxr_mask, which is uint32_t, as uint16_t in the debug
log message. Adding casting to uint16_t before substitution because only
lower 16 bits are significant for debug purposes.

Change-Id: Iddb87cd156dfc84ab1f91cd15a1ddee6b646d412
Signed-off-by: Ilya Kharin <akscram@gmail.com>
Reviewed-on: http://openocd.zylin.com/5590
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>