firmware: qcom: scm: Ensure 'a0' status code is treated as signed

The 'a0' member of 'struct arm_smccc_res' is declared as 'unsigned long',
however the Qualcomm SCM firmware interface driver expects to receive
negative error codes via this field, so ensure that it's cast to 'long'
before comparing to see if it is less than 0.

Cc: <stable@vger.kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>