mtd: spi: renesas: Write DRDMCR register once

Instead of writing DRDMCR with 0 first and then overwriting DRDMCR again
in case any dummy bytes have to be sent out, write DRDMCR in every case
with the amount of dummy bytes that have to be sent out. In case no dummy
bytes have to be sent out, the value written into DRDMCR is zero, so no
dummy bytes are sent out. No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>