ionic: use offset for ethtool regs data

[ Upstream commit f85ae16f ]

Use an offset to write the second half of the regs data into the
second half of the buffer instead of overwriting the first half.

Fixes: 4d03e00a ("ionic: Add initial ethtool support")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>