prism54 - Use offsetof()

Use the standard offsetof() macro to fix a compile warning below:
  CC [M]  drivers/net/wireless/prism54/isl_ioctl.o
drivers/net/wireless/prism54/isl_ioctl.c: In function 'prism2_ioctl_set_generic_element':
drivers/net/wireless/prism54/isl_ioctl.c:2658: warning: cast from pointer to integer of different size
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/prism54/isl_ioctl.c

@@ -2518,7 +2518,7 @@ enum {
 
 #define PRISM2_HOSTAPD_MAX_BUF_SIZE 1024
 #define PRISM2_HOSTAPD_GENERIC_ELEMENT_HDR_LEN \
-((int) (&((struct prism2_hostapd_param *) 0)->u.generic_elem.data))
+	offsetof(struct prism2_hostapd_param, u.generic_elem.data)
 
 /* Maximum length for algorithm names (-1 for nul termination)
  * used in ioctl() */