spi: spi-mem: Fix typo, s/fallback/falls back

"Fallback" in one word is a noun. "Fall back" is two separate words,
a verb and an adverb. Use the latter.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200216214012.1106658-1-tudor.ambarus@microchip.com


Signed-off-by: Mark Brown <broonie@kernel.org>