xtensa: fix TIOCGSERIAL and TIOCSSERIAL definitions

Put equivalent constants into TIOCGSERIAL and TIOCSSERIAL definitions:
size subfield of these IOCTL codes is not used and no other architecture
uses _IO* macros here.

The other way could be putting #include <linux/serial.h> into
net/bluetooth/rfcomm/tty.c.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>