netdev-linux: avoid negative value cast to non-negative in comparison

netdev-linux: avoid negative value cast to non-negative in comparison

I am using a userspace OVS, there is a constant ERR message in log:
"dpif_netdev|ERR|error receiving data from ovs-netdev: Message too long"
Check the code and find there is a comparison between ssize_t and size_t type in
netdev_rx_linux_recv(). It makes the negative "retval" greater than the "size".

Change the sequence of comparison to avoid negative return value cast to
a positive and become greater then a non-negative value.

Signed-off-by: ZhengLingyun <konghuarukhr@163.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>