Interprete mmi code *21*num# as registration.

Per 3GPP TS 22.030 6.5.2

A call forwarding request with a singel * would
be interpreted as registration if containing a
forwarded-to number, or an activation if not.

Change-Id: Iaf5754e49454819892fe054938ef8819f759d6bd
Signed-off-by: Bin Li <libin@marvell.com>