drivers/net/dnet.c: Fix GCC 4.6 warnings

Fix:
dnet.c: In function 'dnet_send':
dnet.c:136:9: warning: variable 'len' set but not used
[-Wunused-but-set-variable]
dnet.c: In function 'dnet_set_hwaddr':
dnet.c:209:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]
dnet.c:209:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]
dnet.c:209:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]
dnet.c:209:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Anatolij Gustschin <agust@denx.de>