thunderbolt: Fix uninitialized variable in tb_tunnel_alloc_usb3()

Currently in case of no bandwidth available for USB3 tunnel, we are left
with uninitialized variable that can lead to huge negative allocated
bandwidth.

Fix this by initializing the variable to zero. While there, fix the
kernel-doc to describe more accurately the purpose of the function
tb_tunnel_alloc_usb3().

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-usb/6289898b-cd63-4fb8-906a-1b6977321af9@moroto.mountain/


Fixes: 25d905d2 ("thunderbolt: Allow USB3 bandwidth to be lower than maximum supported")
Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>