MdeModulePkg/Library/BaseSerialPortLib16550: Ensure FIFO Polled Mode

MdeModulePkg/Library/BaseSerialPortLib16550: Ensure FIFO Polled Mode

Put the UART in FIFO Polled Mode by clearing IER after setting FCR.
Also, add comments to show DLAB state for registers 0 and 1.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leo Duran <leo.duran@amd.com>
Cc: Star Zeng <star.zeng@intel.com>
CC: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>