net/hyperv: Correct the assignment in netvsc_recv_callback()

The first assignment to variable "net" is wrong, but overridden by the
latter assignments. So the bug isn't manifested.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>