serial: 8250: include <linux/serial_reg.h> from serial_8250.h

The header file, include/linux/serial_8250.h, contains references to
UART_LSR_BRK_ERROR_BITS and UART_MSR_ANY_DELTA that are defined in
<linux/serial_reg.h>.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>