SPI: Fix 32 bit transfers in mxc_spi.c

Commit f9b6a157,  "i.MX31: fix SPI
driver for shorter than 32 bit" broke 32 bit transfers. This patch
makes single 32 bit transfer work again.

Transfer lengths that are known not to work will abort and print
an error message.

Tested on i.MX31 Litekit and i.MX31 PDK using 32 bit transfers to
the MC13783/ATLAS chip (using the 'date' command).

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>