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

drivers/spi/spi-mem.c

@@ -487,7 +487,7 @@ static ssize_t spi_mem_no_dirmap_write(struct spi_mem_dirmap_desc *desc,
  * This function is creating a direct mapping descriptor which can then be used
  * to access the memory using spi_mem_dirmap_read() or spi_mem_dirmap_write().
  * If the SPI controller driver does not support direct mapping, this function
- * fallback to an implementation using spi_mem_exec_op(), so that the caller
+ * falls back to an implementation using spi_mem_exec_op(), so that the caller
  * doesn't have to bother implementing a fallback on his own.
  *
  * Return: a valid pointer in case of success, and ERR_PTR() otherwise.