Add enable_ms to jsm driver

This fixes a crash observed when non-existant enable_ms function is
called for jsm driver.

Signed-off-by: Scott Kilau <Scott.Kilau@digi.com>
Signed-off-by: Paul Larson <pl@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>