i40e: add range check to i40e_aq_rc_to_posix

Just to be sure, add a range check to avoid any possible
array index-out-of-bound issues.

CC: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Change-ID: I9323bee6732c2a47599816e1d6c6b3a1f8dcbf54
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Acked-by: Michal Kosiarz <michal.kosiarz@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>