serial: msm_serial_hs: Allow system suspend when no client is active

In some cases, attempt to system resume imbalances the PM usage_count
which completely disables runtime PM callbacks of the device and suspend
doesn't happen.

Allow device to enter suspend through system PM when such condition met
and in addition do not suspend manually if runtime PM is disabled, let
auto suspend takes care of it.

Change-Id: I73df5804d47f551bfee97f43dbe772ee3774b399
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>