serial: Remove extern from func prototypes in headers

Remove unnecessary externs from function prototypes in serial_8250.h
and serial_core.h.

Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230309080923.11778-4-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>