drivers: flash: stm32_qspi: Use sample shift also in single flash mode

The QSPI 1/2 sample shift (SSHIFT) was only enabled in dual flash mode.
This feature is useful to guarantee that data is ready at the sampling
moment, even if the signals are a bit delayed due to PCB constraints.
Therefore, it should be enabled when possible (only supported in STR
mode).

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