spi: Eliminate 3WIRE spi_transfer check

Checking for SPI_3WIRE isn't needed.  spi_setup() already prevents 3WIRE
mode from being combined with DUAL or QUAD mode support.  So there is no
need to differentiate between a single bit device with SPI_3WIRE set and
one with without.  It doesn't change the allowed bit widths.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>