spi: pl022: Add missing error check for devm_kzalloc

Currently this driver is missing a check on the return value of devm_kzalloc,
which would cause a NULL pointer dereference in a OOM situation.

This patch adds a missing check.

Signed-off-by: Kiran Padwal <kiran.padwal@smartplayin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>