ppp: mppe: Use SHA-1 library instead of crypto_shash

Now that a SHA-1 library API is available, use it instead of
crypto_shash.  This is simpler and faster.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250815020705.23055-1-ebiggers@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>