serial: manually inline serial8250_handle_port

Currently serial8250_handle_irq is a trivial wrapper around
serial8250_handle_port, which actually does all the work.

Since there are no other callers of serial8250_handle_port, we
can just move it inline into serial8250_handle_irq.  This also
makes it more clear what functionality any custom IRQ handlers
need to provide if not using serial8250_default_handle_irq.

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>