acer-wmi: Mark acer_wmi_keymap[] as __initconst

sparse_keymap_setup() will make a copy of the keymap, so we can release
the master copy after initialization.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: "Lee, Chun-Yi" <jlee@suse.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>