drivers: mspi_dw: Prevent RX FIFO overflows in Single IO mode

In Single IO mode, where the clock stretching feature is not available,
the RX FIFO could get overflowed if the TX FIFO was filled with more
dummy bytes than the RX FIFO could receive data items at a given moment.
Such problem could be quite easy hit on nRF54H20, for example for longer
RX transfers (like 8 kB) at 8 MHz.
This commit limits accordingly the number of dummy bytes that can be
enqueued in the TX FIFO.

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