octeontx2-af: Return correct CGX RX fifo size

CGX receive buffer size is a constant value and
cannot be read from CGX0 block always since
CGX0 may not enabled everytime. Hence return CGX
receive buffer size from first enabled CGX block
instead of CGX0.

Fixes: 6e54e1c5

 ("octeontx2-af: cn10K: MTU configuration")
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>