drivers: qcom: rpmh: Avoid race when setting solver mode

Ensure that the solver mode is set only when there are no active
requests being processed by the RPMH drivers. Also, since we are no
longer locking in the interrupt handlers, remove the _irqsave variants
of spinlocks.

Change-Id: I61b118c1946488cc95a8f4da3dae975d7361bf46
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>