staging: rtl8712: fix comment

The old comment refers to a typedef name which is being removed,
and to a style of calculation which is no longer being used.
It falsely states that IELength is variable length, instead of IEs.

Change comment to simply state that the IEs field is a buffer of
variable size and that IELength refers to the current size.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>