USB: serial: whiteheat: drop termbits include

The termios definitions are provided by the termios.h header file which
is included by tty.h so drop the redundant asm/termbits.h include
directive.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>