flash/nor/stm32lx: fixed writes at high adapter speeds

flash/nor/stm32lx: fixed writes at high adapter speeds

The busy flag must be polled after each half-page write.
At low clock speeds, no issue is observed when the poll
is omitted, because the writes complete before the next
write begins. But at high clock speeds the subsequent
writes would overlap and cause the operation to fail.

The status polls are done on the target for efficiency,
since the half-pages are very small.

Change-Id: Ia1e9b4a6a71930549b3d84a902744ce6e596301b
Signed-off-by: Jimmy <nhminus@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/5598
Tested-by: jenkins
Reviewed-by: Jelle De Vleeschouwer
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Andrzej Sierżęga <asier70@gmail.com>