Input: wistron_btns - fix a memory leak in wb_module_init error path

select_keymap() calls copy_keymap() to allocate a memory for keymap.
This patch adds a missing kfree(keymap) in wb_module_init error path.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>