mgmt/osdp: Clear secure channel base key from stack

Secure Channel Base Key (SCBK) is a secret key used to derive the session
keys used to encrypt and decrypt OSDP packets. Secure coding practice
requires us to clear such sensitive data from stack once we are done
needing it. This patch addresses this issue.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>