serial: add early_serial_setup() back to header file

early_serial_setup was removed from serial.h, but forgot to put in
serial_8250.h

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>