ixgbe: define IXGBE_LE32_TO_CPUS

Richard Lowe notes in PR 170267 IXGBE_LE32_TO_CPUS was previously
directly defined as le32dec() which is a pure function but the shared
code is expecting an in place conversion.

In SVN r282289 its assignment was removed altogether.

There was some deliberation in the PR on what to define this as, we
just need to do the update in place which is easy enough.

The uintptr_t casts in the shared code were from a DPDK sync and are
unwanted with our new ixgbe_osdep.h implementation.

PR: 170267
Reported by: Richard Lowe <richlowe@richlowe.net>
MFC after: 1 week