hyperv: netvsc: Neaten netvsc_send_pkt by using a temporary

Repeated dereference of nvmsg.msg.v1_msg.send_rndis_pkt can be
shortened by using a temporary.  Do so.

No change in object code.

Miscellanea:

o Use * const for rpkt and nvchan

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>