i40e: Fix inconsistent use of PF/VF vs pf/vf

i40e: Fix inconsistent use of PF/VF vs pf/vf

Joe Perches pointed out that we were inconsistent in the use of
PF vs pf or VF vs vf in our driver code.  Since acronyms are usually
capitalized to denote that it is an acronym, changed all references to
be consistent throughout the code.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>