HID: nintendo: fix unused const warning

JC_RUMBLE_ZERO_AMP_PKT_CNT is only used when force feedback support in
the driver is enabled. Place the declaration in the CONFIG_NINTENDO_FF
ifdef to avoid a warning when compiling without rumble support.

Signed-off-by: Daniel J. Ogorchock <djogorchock@gmail.com>
Reviewed-by: Silvan Jegen <s.jegen@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>