serial: msm_geni_serial: Avoid port pointer access for earlyconsole case

Add defensive check in msm_geni_serial_enable_interrupts to avoid
null pointer dereference issues with earlyconsole usecase.

Change-Id: I7a82a4a387583425c1926a59f7181d39889cba75
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>