spi: omap2-mcspi: Fix FIFO support for transmit-and-receive mode

This patch fixes MCSPI FIFO buffer support when transmit-and-receive
(full duplex) mode is used. In this mode FIFO can be used for RX or
for TX or for both directions. If FIFO used for both directions the buffer
is split into two 32-byte buffers - one for each direction.
Also for full duplex mode both AEL and AFL need to be set in CHCONF0 register.

Signed-off-by: Illia Smyrnov <illia.smyrnov@globallogic.com>
Signed-off-by: Mark Brown <broonie@linaro.org>