serial: export the key functions for an 8250 IRQ handler

For drivers that need to construct their own IRQ handler, the
three components are seen in the current handle_port -- i.e.
Rx, Tx and modem_status.

Make these exported symbols so that "almost" 8250 UARTs can
construct their own IRQ handler with these shared components,
while working around their own unique errata issues.

The function names are given a serial8250 prefix, since they
are now entering the global namespace.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>