spi: au1550: Fix bug in deallocation of memory

This patch fixes a bug on the deallocation of memory allocated using
request_mem_region, by using release_mem_region instead of
release_resource and kfree.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>