ntb: transport: Fix uninitialized mutex

ntb: transport: Fix uninitialized mutex

When the mutex 'link_event_lock' was introduced, it was never
initialized and it triggers kernel warnings when used with locking
debug turned on. Add initialization for the mutex.

Fixes: 3db835dd8f9a ("ntb: Add mutex to make link_event_callback executed linearly.")
Cc: fuyuanli <fuyuanli0722@gmail.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>