mlx5: avoid build warnings on 32-bit

The mlx5 driver passes a string pointer in through a 'u64' variable,
which on 32-bit machines causes a build warning:

drivers/net/ethernet/mellanox/mlx5/core/debugfs.c: In function 'qp_read_field':
drivers/net/ethernet/mellanox/mlx5/core/debugfs.c:303:11: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

The code is in fact safe, so we can shut up the warning by adding
extra type casts.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>