flash/stm32l4x: prevent undefined behavior warnings caused by signed integer operations

flash/stm32l4x: prevent undefined behavior warnings caused by signed integer operations

When running OpenOCD with -fsanitize=undefined, a warning is emitted
for an bit-shifting operation whose result cannot be stored in a
signed integer.

This is because (1 << 31) overflows a signed integer, which is
undefined behavior. By making each of the bit masks act on an
unsigned number, the warning is avoided.

Whether this warning emitted by UBSan would ever manifest into a real
error is debatable, but fixing this does make UBSan happy.

Change-Id: I0455a26b234cb4f5e239a6ba90023d28380e9464
Signed-off-by: Sebastiaan de Schaetzen <sebastiaan.de.schaetzen@gmail.com>
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6429
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Tested-by: jenkins