docbook: don't use serial_core.h in device-drivers book

Fix new kernel-doc warning.  This file no longer contains
kernel-doc comments.

Warning(include/linux/serial_core.h): no structured comments found

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>