UBUNTU: SAUCE: make ASYNCB_INITIALIZED available for the kernel

The flag ASYNCB_INITIALIZED is required by our driver
ubuntu/xr-usb-serial. Make it available to kernel code to prevent the
following build failure:

 ./build/ubuntu/xr-usb-serial/xr_usb_serial_common.c:1613:15: error: 'ASYNCB_INITIALIZED' undeclared (first use in this function); did you mean 'RCU_INITIALIZER'?
  1613 |  if (test_bit(ASYNCB_INITIALIZED, &xr_usb_serial->port.flags))
       |               ^~~~~~~~~~~~~~~~~~
       |               RCU_INITIALIZER

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

include/uapi/linux/tty_flags.h

@@ -32,6 +32,7 @@
 #define ASYNCB_AUTOPROBE	15 /* [x] Port was autoprobed by PCI/PNP code */
 #define ASYNCB_MAGIC_MULTIPLIER	16 /* Use special CLK or divisor */
 #define ASYNCB_LAST_USER	16
+#define ASYNCB_INITIALIZED	31 /* Serial port was initialized */
 
 /*
  * Internal flags used only by kernel (read-only)
@@ -40,7 +41,6 @@
  *	    TTY_PORT_ flags in the iflags field (and not userspace-visible)
  */
 #ifndef __KERNEL__
-#define ASYNCB_INITIALIZED	31 /* Serial port was initialized */
 #define ASYNCB_SUSPENDED	30 /* Serial port is suspended */
 #define ASYNCB_NORMAL_ACTIVE	29 /* Normal device is active */
 #define ASYNCB_BOOT_AUTOCONF	28 /* Autoconfigure port on bootup */