scsi: introduce shost_priv helper

Currently accessing the scsi host private data is rather messy because it comes as an unsigned long that always needs a cast first. This patch introduces a helper that does the cast called shost_priv. It's similar in spirit to netdev_priv for network drivers.

This is the first patch introducing the macro, and the second patch in the series will convert esp and it's subdrivers as an example. Further conversion will wait until the helper is in the tree to make patch juggling easier.

Index: linux-2.6/include/scsi/scsi_host.h
===================================================================
@@ -677,6 +677,10 @@ struct Scsi_Host {
 #define shost_printk(prefix, shost, fmt, a...)	\
 	dev_printk(prefix, &(shost)->shost_gendev, fmt, ##a)
 
+static inline void *shost_priv(struct Scsi_Host *shost)
+{
+	return (void *)shost->hostdata;
+}
 
 int scsi_is_host_device(const struct device *);