serial: core: Fix the definition name in the comment of UPF_* flags

From day 1 the UPF_LAST_USER wasn't defined, a specific number of
the last bit for userspace. Instead the code always relies on
ASYNCB_LAST_USER. Fix comment accordingly.

Fixes: 904326ecac ("tty,serial: Unify UPF_* and ASYNC_* flag definitions")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220203144521.16457-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_core.h

@@ -171,7 +171,7 @@ struct uart_port {
 	 * assigned from the serial_struct flags in uart_set_info()
 	 * [for bit definitions in the UPF_CHANGE_MASK]
 	 *
-	 * Bits [0..UPF_LAST_USER] are userspace defined/visible/changeable
+	 * Bits [0..ASYNCB_LAST_USER] are userspace defined/visible/changeable
 	 * The remaining bits are serial-core specific and not modifiable by
 	 * userspace.
 	 */