ionic: replace set_vf data with union

This (ab)use of a data buffer made some static code checkers
rather itchy, so we replace the a generic data buffer with
the union in the struct ionic_vf_setattr_cmd.

Fixes: fbb39807 ("ionic: support sr-iov operations")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>