firmware_loader: use SHA-256 library API instead of crypto_shash API

firmware_loader: use SHA-256 library API instead of crypto_shash API

This user of SHA-256 does not support any other algorithm, so the
crypto_shash abstraction provides no value.  Just use the SHA-256
library API instead, which is much simpler and easier to use.

Also take advantage of printk's built-in hex conversion using %*phN.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250428190909.852705-1-ebiggers@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>