staging: ozwpan: Fix farewell report.

This patch fix following issues reported by Dan:-

1) There is no check limiting the size to 32 and it could be up to
   253 bytes.
2) Use defines instead of magic numbers.
3) The oz_farewell struct is supposed to be a variable length struct
   but the variable part is put in the middle.  It doesn't make any
   sense to put the length of the variable size array after then end
   of the array because we can never find it again!  Put the
   variable size array at the end.  Make it a zero length array.
   u8 len;
   u8 report[0];
4) In oz_add_farewell() we do this:

	f = kmalloc(sizeof(struct oz_farewell) + len - 1, GFP_ATOMIC);

    The "- 1" refers to sizeof(f->report) but because it was a magic
    number then it was missed when the sizeof(f->report) changed.
5) In [patch 6/6] we set the ->len member.  But because it is at the
   end of a variable length array with no limit check the remote
   attacker can just rewrite it using the memcpy() on the next line.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rupesh Gujare <rupesh.gujare@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>