drivers: flash: stm32_qspi: Factorize all status register reads

The 'qspi_read_status_register' routine implements the reading of a
flash memory's status register. This routine is used anytime reading a
status register is needed, except in 'qspi_wait_until_ready'. This
commit moves the read routine to be able to use it in
'qspi_wait_until_ready'. The 'qspi_write_status_register' is also moved
to keep it close to the read routine.

Signed-off-by: Thomas Altenbach <altenbach.thomas@gmail.com>