octeontx2-af: RPM: Fix low network performance

Low network performance is observed even on RPMs with larger
FIFO lengths.

The cn10kb silicon has three RPM blocks with the following
FIFO sizes:

         --------------------
         | RPM0  |   256KB  |
         | RPM1  |   256KB  |
         | RPM2  |   128KB  |
         --------------------

The current design stores the FIFO length in a common structure for all
RPMs (mac_ops). As a result, the FIFO length of the last RPM is applied
to all RPMs, leading to reduced network performance.

This patch resolved the problem by storing the fifo length in per MAC
structure (cgx).

Fixes: b9d0fedc ("octeontx2-af: cn10kb: Add RPM_USX MAC support")
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>