mtd: spi: renesas: Configure DRDRENR register

Make sure DRDRENR register is configured before performing external
address space read. This register might have been configured by a
prior stage bootloader and leaving it unconfigured would interfere
with U-Boot operation. Since U-Boot RPC SPI driver does not support
DDR data transfer mode yet, set this register unconditionally to 0.

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