usb: usbtmc: Fix memory leak in usbtmc_ioctl_request

Kernel memory is allocated twice in new function
usbtmc_ioctl_request and creates a memory leak.
This fix removes the superfluous kmalloc().

Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Fixes: 658f24f4 ("usb: usbtmc: Add ioctl for generic requests on control")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>