USB: BUG_ON conversion for wacom.c

this patch converts two if () BUG(); construct to BUG_ON();
which occupies less space, uses unlikely and is safer when
BUG() is disabled.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Acked-by: "Ping Cheng" <pingc@wacom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>