drivers: flash_mspi_nor: Omit quad_enable_set() when QER is set to NONE

When the quad-enable-requirements property is set to "NONE" or is not
present, no Quad Enable operation should be performed.
This fixes an issue with the mx25uw6345g flash chip that is present
on the nRF54h20 DK and supports the Single I/O mode, but cannot be
used in that mode.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>