Fix uninitialized symbol 'actual' in function usbtmc_ioctl_clear.

When symbol 'actual' is not initialized and usb_bulk_msg() fails,
the subsequent kernel debug message shows a random value.
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Fixes: dfee02ac ("usb: usbtmc: Fix ioctl USBTMC_IOCTL_CLEAR")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>