drivers: soc: qcom: rpmh-rsc: Correct check for slot number

The return index value from bitmap_find_next_zero_area can be higher
than available slot. So correct the check to return error in such case.

Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>
Reviewed-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Andy Gross <agross@kernel.org>