Remove TLS-related inlines from t4_tom.h to fix iw_cxgbe(4) build.

• Remove the one use of is_tls_offload() and the function. AIO special handling only needs to be disabled when a TOE socket is actively doing TLS offload on transmit. The TOE socket's mode (which affects receive operation) doesn't matter, so remove the check for the socket's mode and only check if a TOE socket has TLS transmit keys configured to determine if an AIO write request should fall back to the normal socket handling instead of the TOE fast path.
• Move can_tls_offload() into t4_tls.c. It is not used in critical paths, so inlining isn't that important. Change return type to bool while here.

Sponsored by: Chelsio Communications