USB: usbtmc: fix switch statment

Steve Holland pointed out that we forgot to call break; in the switch
statment.  This probably resolves a lot of the bug reports I've gotten
for the driver lately.

Stupid me...

Reported-by: Steve Holland <sdh4@iastate.edu>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>