iavf: Enable flex-bytes support

Flex-bytes allows for packet matching based on an offset and value. This
is supported via the ethtool user-def option.

The user-def 0xAAAABBBBCCCCDDDD: BBBB is the 2 byte pattern while AAAA
corresponds to its offset in the packet. Similarly DDDD is the 2 byte
pattern with CCCC being the corresponding offset. The offset ranges from
0x0 to 0x1F7 (up to 504 bytes into the packet). The offset starts from
the beginning of the packet.

This feature can be used to allow customers to set flow director rules
for protocols headers that are beyond standard ones supported by ethtool
(e.g. PFCP or GTP-U).

Like for matching GTP-U's TEID value 0x10203040:
ethtool -N ens787f0v0 flow-type udp4 dst-port 2152 \
user-def 0x002e102000303040 action 13

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Tested-by: Chen Bo <BoX.C.Chen@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>