[ALPS08719090] uart: mt6989: check APDMA tx addr

Add a check for the validity of the ADMA address before
starting apdmas start_tx.

MTK-Commit-Id: 5757c4897b65d532aa15198fc9d335fe882f8283

Signed-off-by: lucky.dong <lucky.dong@mediatek.com>
CR-Id: ALPS08719090
Feature: uart_driver
Change-Id: I9a11868df328d0a6d208570e6156965fd0955cf7