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

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: 904326ecac02 ("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>