ethdev: update flow item GTP QFI definition

ethdev: update flow item GTP QFI definition

'qfi' field is 8 bits which represent single bit for
PPP (paging Policy Presence) single bit for RQI
(Reflective QoS Indicator) and 6 bits for QFI
(QoS Flow Identifier)
This is based on RFC 38415-g30
https://www.3gpp.org/ftp/Specs/archive/38_series/38.415/38415-g30.zip

Updated the doxygen comment and the mask for 'qfi'
to properly identify the full 8 bits of the field.

note: changing the default mask would cause different
patterns generated by testpmd.

Fixes: 346553db5bd1 ("ethdev: add GTP extension header to flow API")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@nvidia.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>