drivers/net/skfp/pmf.c: Use offsetof() macro

drivers/net/skfp/pmf.c: Use offsetof() macro

Self-baked macros cause bunch of compile warnings like below:

  CC [M]  drivers/net/skfp/pmf.o
  CC      net/ipv4/fib_semantics.o
drivers/net/skfp/pmf.c:86: warning: cast from pointer to integer of different size
drivers/net/skfp/pmf.c:87: warning: cast from pointer to integer of different size
...

Use the standard offsetof() macro instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>