drivers: mspi_dw: Remove needless TXEIR check

This is a follow-up to commit f0f5f8cdef74c79e2f28048dd842a05a1241e5af.

There is no need to check if TXE interrupt flag is set before
calling tx_dummy_bytes(), as the function can handle the case
when it is called even though there is no room in the TX FIFO.
On the other hand, the check may be actually harmful, as it may
prevent adding more items to the TX FIFO while the SSI controller
is waiting until the FIFO achieves its transfer start level.
Remove the check then and exit the ISR loop when no dummy bytes
could be written into the TX FIFO.

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