serial: msm_geni_serial: Use IS_ENABLED() instead of ifdef CONFIG

The IS_ENABLED() macro checks if a Kconfig symbol has been enabled either
built-in or as a module, use that macro instead of open coding the same.

Change-Id: I20ba557462b3080483e60ffd68a2e50d79468135
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>