staging: et131x: Use register defines instead of magic numbers

Use register name defines instead of magic numbers where a bit of
clarity would be useful.

Also a small typo fix and some register prefixes added, for further
clarity.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>