serial: 8250: Refactor serial8250_rx_chars() inner loop

Factor the read/process one char inner loop to a separate helper
function serial8250_read_char(). No functional change.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>