mtd: m25p80: remove obsolete FIXME

The FIXME and NOTE have already been fixed (we have FAST_READ support).

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Sourav Poddar <sourav.poddar@ti.com>
Acked-by: Marek Vasut <marex@denx.de>