random: clamp credited irq bits to maximum mixed

Since the most that's mixed into the pool is sizeof(long)*2, don't
credit more than that many bytes of entropy.

Fixes: e3e33fc2 ("random: do not use input pool from hard IRQs")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>