usb: core: fix memory leak on port_dev_path allocation

Currently the allocation of port_dev_path from the call to
kobject_get_path is not being kfree'd, causing a memory leak. Fix
this by kfree'ing this at the end of the function. Add an extra
error exit path to fix one of the early leaks when envp[0] fails
to be allocated.

Detected by CoverityScan, CID#1473771 ("Resource Leak")

Fixes: 201af55d

 ("usb: core: added uevent for over-current")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>