hwspinlock: Convert to use 'switch' statement

We have different hwspinlock modes to select, thus it will be more
readable to handle different modes with using 'switch' statement
instead of 'if' statement.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>