[PATCH] hostap: use offsetof() instead of own equivalent

The original macros result in gcc 4.2 warning about "cast from pointer
to integer of different size" on 64-bit systems.

Use of offsetof() on fields in substructures is widespread throughout
the kernel code and should work whether offsetof() is defined using
__compiler_offsetof() or a cast.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>