USB: usbtmc: Bump USBTMC_API_VERSION value

The previous patches in this series have changed the behaviour of the
driver and added new calls.

Tested-by: Jian-Wei Wu <jian-wei_wu@keysight.com>
Reviewed-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20201215155621.9592-5-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>