serial: msm_geni_serial: Remove conditional check from earlycon

At the time of early console, probe time variable will not be
available. Hence remove the check and readback register directly.

Change-Id: I8b37abb576de87306a9361c49de44bd225f9e55e
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>