octeontx2-pf: fix FLOW_DIS_IS_FRAGMENT implementation

Upon reviewing the flower control flags handling in
this driver, I notice that the key wasn't being used,
only the mask.

Ie. `tc flower ... ip_flags nofrag` was hardware
offloaded as `... ip_flags frag`.

Only compile tested, no access to HW.

Fixes: c672e372

 ("octeontx2-pf: Add support to filter packet based on IP fragment")
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>