usbip: dynamically allocate idev by nports found in sysfs

As the amount of available ports varies by the kernels build
configuration. To remove the limitation of the fixed 128 ports
we allocate the amount of idevs by using the number we get
from the kernel.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Acked-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>