msm: clock-8974: Fix modem clock measurement code

Touching the MSS CC registers without a certain clock being on
results in the writes not going through and L2 controller errors.
Callers of measurement should ensure that the clock
(gcc_mss_cfg_ahb_clk) is on, therefore, don't touch the MSS CC
measurement related registers when measuring clocks in other CCs.

While we're here, correct a measurement mux index for an MSS clock.

Change-Id: Ic883cba9ff91efc5f7c65ec1f489c000eca229ea
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>