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

Return index value from bitmap_find_next_zero_area can be higher
than available slot. Correct check for same.

Change-Id: I544937c31e03b582a08f73c1d7013060aaa2e654
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>