platform/x86: dell-laptop: fix kbd_get_state's request value

Commit 9862b436 ("platform/x86: dell-laptop: Allocate buffer on heap
rather than globally")
broke one request, changed it back to the original value.

Tested on a Dell E6540, backlight came back.

Fixes: 9862b436

 ("platform/x86: dell-laptop: Allocate buffer on heap rather than globally")
Signed-off-by: Laszlo Toth <laszlth@gmail.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>