[ril/rilmodem] Using pdu field to figure out SMSC length (LP: #1090437).
Adding comment on how calculation of smsc length is done.
[ril/rilmodem] Using pdu field to figure out SMSC length (LP: #1090437). Fixes: https://bugs.launchpad.net/bugs/1090437.

Approved by Ricardo Salveti, PS Jenkins bot, Tony Espy.