net: fec: limit register access on i.MX6UL

net: fec: limit register access on i.MX6UL

Using 'ethtool -d […]' on an i.MX6UL leads to a kernel crash:

   Unhandled fault: external abort on non-linefetch (0x1008) at […]

due to this SoC has less registers in its FEC implementation compared to other
i.MX6 variants. Thus, a run-time decision is required to avoid access to
non-existing registers.

Fixes: cb47babb656b ("net: fec: use a more proper compatible string for i.MX6UL type device")
Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20221024080552.21004-1-jbe@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>