spi: armada-3700: Replaced raw values for nbits by the SPI macros

Currently, function a3700_spi_pin_mode_set() configures the SPI transfer
mode according to the value passed as second argument. This value is
detected using the raw values from a switch case.

This commit replaces these raw values by the corresponding macro
constants in linux/spi/spi.h

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>