netdevsim: fix ptr_ret.cocci warnings

drivers/net/netdevsim/dev.c:937:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: 6556ff32

 ("netdevsim: use IS_ERR instead of IS_ERR_OR_NULL for debugfs")
CC: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>