drivers: serial: cdns: fix interrupt driven uart

The tx fifo empty interrupt is a edge driven interrupt, so if it
is already empty then and the interrupt is enabled, it will not
fire so the isr needs to be triggered manually for the callback.

This also removes the unnecessary interrupt locking in the isr and
removes the receiver timeout interrupt.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>