crypto: qce - switch to using a mutex

Having switched to workqueue from tasklet, we are no longer limited to
atomic APIs and can now convert the spinlock to a mutex. This, along
with the conversion from tasklet to workqueue grants us ~15% improvement
in cryptsetup benchmarks for AES encryption.

While at it: use guards to simplify locking code.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>