thunderbolt: Log path activation failures without WARN backtraces

thunderbolt: Log path activation failures without WARN backtraces

tb_path_activate() currently logs failures with tb_WARN(), which triggers
a stack trace. Transient conditions such as lane bonding or Type-C link
hiccups can fail path activation briefly, and the resulting backtraces are
noisy without aiding diagnosis.

Switch to tb_warn() for all path activation failures. The error code is
already returned to callers, and the warning still shows the message
without the backtrace.

Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>