mtd: m25p80: Set rx_nbits for Quad SPI transfers

When using the Quad Read opcode, SPI masters still use Single SPI
transfers, as spi_transfer.rx_nbits defaults to SPI_NBITS_SINGLE.
Use SPI_NBITS_QUAD to fix this.

While an earlier version of commit 3487a639
("drivers: mtd: m25p80: add quad read support") did this correctly, it was
forgotten in the version that got merged.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>