platform/x86: mlx-platform: Fix access mode for fan_dir attribute

Fix access mode for "fan_dir" attribute from "write only" to
"read only". This attribute is exposed to leds-mlxreg driver.
The purpose of this attribute is to provide information about FAN
direction setting on the system (forward or backward).
It is relevant for the next generation systems MQMB7xx, MSN37xx,
MSN34xx, MSN38xx.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>