spi: atmel-quadspi: fix resume call

When waking up from the Suspend-to-RAM state, the following error
was seen:

m25p80 spi2.0: flash operation timed out

The flash remained in an undefined state, returning 0xFFs.
Fix it by setting the Serial Clock Baud Rate, as it was set
before the conversion to SPIMEM.

Tested with sama5d2_xplained and mx25l25673g spi-nor in
Backup + Self-Refresh and Suspend modes.

Fixes: 0e6aae08 ("spi: Add QuadSPI driver for Atmel SAMA5D2")
Reported-by: Mark Deneen <mdeneen@gmail.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>