net: stmmac: Remove some unnecessary void pointers

There's a few spots in the hardware interface where a void pointer is
used, but what's passed in and later cast out is always the same type.

Just use the proper type directly.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>