platform_data/mlxreg: Document fixes for core platform data

Remove "led" from the description, since the structure
"mlxreg_core_platform_data" is used not only for led data.

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