ipw2200: prevent alloc of unspecified size on stack

if log_len is larger than 4K then we are killing the stack.
allocate on heap instead and limit size to what practically can
be used (PAGE_SIZE)

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>