serial: omap8250: Simplify rx dma completion interface

Extract the operation (restarting new rx dma) performed when error
argument is true.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>