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>