usb: usbtmc: Add ioctl USBTMC_IOCTL_CANCEL_IO

ioctl USBTMC_IOCTL_CANCEL_IO stops and kills all flying urbs of
last USBTMC_IOCTL_READ and USBTMC_IOCTL_WRITE function calls.
A subsequent call to USBTMC_IOCTL_READ or
USBTMC_IOCTL_WRITE_RESULT returns -ECANCELED with
information about current transferred data.

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>