i40e: shorten wordy fields

The alloc_rx_buff_failed and alloc_rx_page_failed variables
are both part of an rx specific structure so just remove
the _rx part of the name.  No functional changes.

Change-ID: Icffa2f5d13c6f2b1e09cf45b9472b83c9dae8fc6
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>