flash/stm32h7x: fix FLASH_WPSN_PRG mask used for protection

flash/stm32h7x: fix FLASH_WPSN_PRG mask used for protection

STM32H7Ax/7Bx devices have a different WPSN mask (0xFFFFFFFF),
(0xFF for STM32H74x/75x and STM32H72x/73x devices).

And when supporting STM32H7Ax/7Bx devices, stm32x_protect() was
not updated accordingly.

Change-Id: I081217af3e5ed815b67bfdfec7f4ebaa3152a865
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Fixes: 0b7eca17691a (flash/stm32h7x: add support of STM32H7Ax/H7Bx devices)
Reviewed-on: https://review.openocd.org/c/openocd/+/6858
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins