Revert "driver: uart: npcx: add missing tx/rx interrupt enabled checks"

This reverts commit 271b306b.

In rare conditions, this commit causes the uart shell mechanism to
corrupt on Cros ec system. Revert this commit on upstream repo first to
avoid blocking the development. Will dig out why this symptom occurs
later.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>