serial: Refactor code to reduce cyclomatic complexity

Resolve the cyclomatic complexity numbers by refactoring the code.

Fixes: 39fc927ed715 ("serial: Propagate missing UART fix 4.19 to 5.4")
Change-Id: I599900246852e002d68364affc8ff032a6231324
Signed-off-by: Akash Asthana <akashast@codeaurora.org>