i40evf: make comparisons consistent

Most of the null-checking in this driver is of the style if (!foo),
except these few. Make these checks consistent with the rest of the
code.

Change-ID: I991924f34072fa607a1b626a8b3f1fa5195d43e9
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>