drivers: net: smc911x: Fix cast from pointer to integer of different size

drivers/net/ethernet/smsc/smc911x.c: In function ‘smc911x_hardware_send_pkt’:
drivers/net/ethernet/smsc/smc911x.c:471:11: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
  471 |  cmdA = (((u32)skb->data & 0x3) << 16) |

When built on 64bit targets, the skb->data pointer cannot be cast to a
u32 in a meaningful way. Use uintptr_t instead.

Suggested-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>