cxgbe: De-duplicate some of the code for managing TLS key contexts.

The NIC TLS and TOE TLS modes in cxgbe(4) both work with TLS key
contexts. Previously, TOE TLS supported TLS key contexts created by
two different methods, and NIC TLS had a separate bit of code copied
from NIC TLS but specific to KTLS. Now that TOE TLS only supports
KTLS, pull common code for creating TLS key contexts and programming
them into on-card memory into t4_keyctx.c.

Sponsored by: Chelsio Communications