Add ioctl USBTMC_IOCTL_API_VERSION to get current API version
of usbtmc driver.

This is to allow an instrument library to determine whether
the driver API is compatible with the implementation.

The API may change in future versions. Therefore the macro
USBTMC_API_VERSION should be incremented when changing tmc.h
with new flags, ioctls or when changing a significant behavior
of the driver.
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Reviewed-by: Steve Bayless <steve_bayless@keysight.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>