iavf: define Rx descriptors as qwords

The union iavf_32byte_rx_desc consists of two unnamed structs defined
inside. One of them represents legacy 32 byte descriptor and second the
16 byte descriptor (extended to 32 byte). Each of them consists of
bunch of unions, structs and __le fields that represent specific fields
in descriptor.

This commit changes the representation of iavf_32byte_rx_desc union
to store four __le64 fields (qw0, qw1, qw2, qw3) that represent
quad-words. Those quad-words will be then accessed by calling
leXY_get_bits macros in upcoming commits.

Suggested-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>