SMC91x: Fix compilation on some platforms.

This reverts 51ac3bef ('SMC91x: delete
unused local variable "lp"') and adds __maybe_unused markers to these
(potentially) unused variables.

The issue is that in some configurations SMC_IO_SHIFT evaluates
to '(lp->io_shift)', but in some others it's plain '0'.

Based upon a build failure report from Manuel Lauss.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/smc91x.c

@@ -2060,6 +2060,7 @@ static int smc_request_attrib(struct platform_device *pdev,
 			      struct net_device *ndev)
 {
 	struct resource * res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "smc91x-attrib");
+	struct smc_local *lp __maybe_unused = netdev_priv(ndev);
 
 	if (!res)
 		return 0;
@@ -2074,6 +2075,7 @@ static void smc_release_attrib(struct platform_device *pdev,
 			       struct net_device *ndev)
 {
 	struct resource * res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "smc91x-attrib");
+	struct smc_local *lp __maybe_unused = netdev_priv(ndev);
 
 	if (res)
 		release_mem_region(res->start, ATTRIB_SIZE);