USB: usbtmc: Change magic number to constant

These patches implement a modification of the USBTMC
protocol to allow operation with Rigol equipment. Cosmetic change to show
that 12 is the USBTMC header size.

Signed-off-by: Alexandre Peixoto Ferreira <alexandref75@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>