drivers: net: tc6: Saturate RCA and RBA when read directly from OA_BUFSTS

The RCA and RBA fields in OA_BUFSTS register are stored with 8 bits each.

On the other hand, when one receives those values in footer, the value
is saturated to 5 bits due to 32 bit size constrain of the footer itself.

To avoid any mismatches - the values read from OA_BUFSTS are saturated
as well.

Signed-off-by: Lukasz Majewski <lukma@denx.de>