net/net.c: cosmetic: do not use assignment in if condition

This removes the following checkpatch issue:
 - ERROR: do not use assignment in if condition

Signed-off-by: Luca Ceresoli <luca.ceresoli@comelit.it>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Ben Warren <biggerbadderben@gmail.com>