Staging: hv: netvsc_drv: Clean up the netvsc_drv_exit() function

To conform to the Linux device model, the device should persist even
when there is no driver bound to it. Cleanup the netvsc_drv_exit() routine
keeping this in mind.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Abhishek Kane <v-abkane@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>