octeontx2-af: add option to toggle DROP_RE enable in rx cfg

Add option to toggle DROP_RE bit in rx cfg mbox. This helps in
modifying the config runtime as opposed to setting available via
nix_lf_alloc() mbox at NIX LF init time.

Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Signed-off-by: Jerin Jacob Kollanukkaran <jerinj@marvell.com>
Signed-off-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>