serial: fix missing bit coverage of ASYNC_FLAGS

It seems that currently ASYNC_FLAGS is one bit short of covering all the
bits of the ASYNC user flags.  In particular it does not cover the
ASYNC_AUTOPROBE bit.

ASYNCB_LAST_USER and ASYNCB_AUTOPROBE are both equal to 15.

Therefore:
ASYNC_AUTOPROBE = 1000 0000 0000 0000
ASYNC_FLAGS     = 0111 1111 1111 1111

So ASYNC_FLAGS is not covering the ASYNC_AUTOPROBE bit.

This patch fixes the issue and with the patch the values will be:
ASYNC_AUTOPROBE = 1000 0000 0000 0000
ASYNC_FLAGS     = 1111 1111 1111 1111

As a side note, doing a "git grep" I didn't find any use of
ASYNC_AUTOPROBE or ASYNCB_AUTOPROBE in the kernel, besides this include
file.

Signed-off-by: John Villalovos <john.l.villalovos@intel.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>