usb: usbip tool: Fix get_nports()

The commit 0775a9cb ("usbip: vhci extension: modifications
to vhci driver") introduced multiple controllers, and nports as a sys
file, and claimed to read the nports from it, but it didn't.

In addition, the get_nports() has been so wrong that even with 8 port
lines for instance, it gets 7 (I am guessing it is due to a '\n' mess).
Nevertheless, we fix it by reading the nports attribute.

Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>