spi: mxs: Use list_for_each_entry to iterate over transfer list

We don't modify the list entry while iterating the transfer list.
So use list_for_each_entry instead of list_for_each_entry_safe.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>