usb: usbtmc: uninitialized symbol 'actual' in usbtmc_read

Fix uninitialized symbol 'actual' in function usbtmc_read.

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: d7604ff0 ("usb: usbtmc: Optimize usbtmc_read")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>