netvsc: use refcount_t for keeping track of sub channels

Rather than a lock and variable, use a refcount_t to keep track
of the number of sub channels.  Don't need to wait for subchannels
on device removal since wait was already done in device_add.

Also fix the error handling; don't wait forever in case of
an error on request to create sub channels.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>