tty: serial: 8250: omap: add custom irq handling

We have (or will have) custom DMA callbacks in the omap driver due to
the different behaviour in the RX and TX case. To make this work
we need a few changes in the IRQ handler to invoke the rx_handler again
after the "manual" mode or retry the tx_handler again before falling
back to the manual mode.

Heikki didn't want to see the extra hacks in the generic / default irq
handler and Peter wasn't too happy about an OMAP-only IRQ handler. The
way I planned it is to use this extra IRQ routine only in DMA case. If
Peter dislike this approach then I hope Heikki doesn't block changes in
the default IRQ handler :)

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>