serial: core: move UPF_NO_TXEN_TEST to quirks and rename

First 16 bits in the flags field are user-visible except
UPF_NO_TXEN_TEST. To keep it clean we introduce internal quirks and move
UPF_NO_TXEN_TEST to them. Rename the constant to UPQ_NO_TXEN_TEST to
distinguish with port flags. Users are converted accordingly.

The quirks field might be extended later to hold the additional ones.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>