Input: Fix spelling mistake in Kconfig "useable" -> "usable"

There is a spelling mistake in the Kconfig text. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210705100230.7583-1-colin.king@canonical.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>