at91_emac: fix compile warning

This patch removes the warning

---8<---
at91_emac.c: In function 'at91emac_write_hwaddr':
at91_emac.c:487:2: warning: dereferencing type-punned pointer will break strict-aliasing rules
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@gmail.com>