USB: serial: drop termios-flag debugging

Drop some unnecessary termios-flag debugging that have been faithfully
reproduced in a few old drivers, including the "clfag" typo and all.

This also addresses a compiler warning on sparc where tcflag_t is
unsigned long and would have required an explicit cast.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>