counter: rz-mtu3-cnt: Unlock on error in rz_mtu3_count_ceiling_write()

These error paths need to call mutex_unlock(&priv->lock) before
returning.  The lock is taken in rz_mtu3_lock_if_counter_is_valid().

Fixes: 25d21447d896 ("counter: Add Renesas RZ/G2L MTU3a counter driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/7b535d6b-6031-493a-84f6-82842089e637@kili.mountain