cxgb4vf: Behave properly when CONFIG_DEBUG_FS isn't defined ...

When CONFIG_DEBUG_FS we get "ERR_PTR()"s back from the debugfs routines
instead of NULL.  Use the right predicates to check for this.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>