ixgbevf: fix Rx function selection

ixgbevf: fix Rx function selection

The logic to select ixgbe VF RX function is different than PF side.

There are a few issues with its current state:
 - it does not allow to select ixgbe_recv_pkts_vec among other options.
 - it can cause memory corruption for scatter mode as it does not allocate
   enough entries in sw_ring.
 - when checksum is enabled, incorrect vector RX function is selected.

To solve above issues, change the VF RX function selection logic to
mimic PF side.

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>