net: enetc: cache accesses to &priv->si->hw

The &priv->si->hw construct dereferences 2 pointers and makes lines
longer than they need to be, in turn making the code harder to read.

Replace &priv->si->hw accesses with a "hw" variable when there are 2 or
more accesses within a function that dereference this. This includes
loops, since &priv->si->hw is a loop invariant.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>