ionic: use offset for ethtool regs data

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>