drivers: qcom: rpmh-rsc: optimize redundant read to determine TCS idle

Since the tcs_in_use flag is authoritative enough to determine the
idleness of the TCS, avoid reading the register redundantly.

Change-Id: Ic7737e60ca0029b3cf9d1cc9e613435608be7bc8
Signed-off-by: Lina Iyer <ilina@codeaurora.org>