ixgbe: cleanup ixgbe_alloc_rx_buffers

This change re-orders alloc_rx_buffers to make better use of the packet
split enabled flag.  The new setup should require less branching in the
code since now we are down to fewer if statements since we either are
handling packet split or aren't.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>