ARM: footbridge: fix EBSA285 LEDs

- The LEDs register is write-only: it can't be read-modify-written.
- The LEDs are write-1-for-off not 0.
- The check for the platform was inverted.

Fixes: cf6856d6

 ("ARM: mach-footbridge: retire custom LED code")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: stable@vger.kernel.org