firmware: qcom_scm: Cleanup code in qcom_scm_assign_mem()

firmware: qcom_scm: Cleanup code in qcom_scm_assign_mem()

There are some questionable coding styles in this function. It looks
quite odd to deref a pointer with array indexing that only uses the
first element. Also, destroying an input/output variable halfway through
the function and then overwriting it on success is not clear. It's
better to use a local variable and the kernel macros to step through
each bit set in a bitmask and clearly show where outputs are set.

Cc: Ian Jackson <ian.jackson@citrix.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Avaneesh Kumar Dwivedi <akdwived@codeaurora.org>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
[bjorn: Changed for_each_set_bit() size to BITS_PER_LONG]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>