drivers: flash: Fix timeout handling in STM32 flash driver

Fix timeout error that can occur in rare case.
When the thread writing to flash is pre-emptive it can be scheduled
out after reading the status register, but before checking if timeout
has expired. In this case it will report timeout without re-checking
the status register.

When writing a lot to flash, for example a firmware update process
then this situation is very likely to occur.

Signed-off-by: Joakim Andersson <joerchan@gmail.com>