net/ibm/emac: Remove VLA usage

net/ibm/emac: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
removes the VLA used for the emac xaht registers size. Since the size
of registers can only ever be 4 or 8, as detected in emac_init_config(),
the max can be hardcoded and a runtime test added for robustness.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Christian Lamparter <chunkeey@gmail.com>
Cc: Ivan Mikhaylov <ivan@de.ibm.com>
Cc: netdev@vger.kernel.org
Co-developed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>