i40evf: don't overflow buffer

If the user adds an obscene amount of MAC addresses, the driver will run
into the situation where it has too many address requests to fit into a
single PF message. The driver checks for this case, and calculates the
maximum number of messages that it can send. Then it completely ignores
this count and overflows the buffer.

Fix this by checking the address count and bailing out of the loop at
the appropriate time.

Change-ID: If8dcbb04602c75941dc0cd8309065e1de9ca791c
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>