clk: qcom: rpmh: Simplify clk_rpmh_bcm_send_cmd()

This function has some duplication in unlocking a mutex and returns in a
few different places. Let's use some if statements to consolidate code
and make this a bit easier to read.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
CC: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20200309221232.145630-2-sboyd@kernel.org