mxs/spi: Rework the mxs_ssp_timeout to be more readable

Rework the mxs_ssp_timeout() function to make it a bit more readable
and hopefully less error prone. Also, have only one successful exit
from the function and one failing exit instead of two.

Finally, discard the udelay() from this function altogether, as this
tightloop is quick enough it's pointless.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>