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

Fix:
mvgbe.c: In function 'mvgbe_send':
mvgbe.c:555:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]
mvgbe.c: In function 'mvgbe_recv':
mvgbe.c:640:2: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Acked-By: Prafulla Wadaskar <prafulla@marvell.com>