tools:iio:generic_buffer: fix order of freeing data

data gets allocated before buffer_access, so it should be freed in reverse
order. Otherwise, if allocating buffer_access fails, an attempt to free it
would be taken, which should not happen.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>