ice: Bug fixes in ethtool code

1) Return correct size from ice_get_regs_len.
2) Fix incorrect use of ARRAY_SIZE in ice_get_regs.

Fixes: fcea6f3d

 (ice: Add stats and ethtool support)
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>