Samsung STK: Fix SMS-based STK

SEND_SMS was falling through to SEND_USSD (which used to do nothing
until USSD support was added) causing an Exception because cmdParams
is not an instance of SendUSSDParams.

Fix by using type-checking instead of checking if Samsung STK is
enabled (because SendSMSParams and SendUSSDParams will only be
created if Samsung STK is enabled.)

Change-Id: I671014e295e9e529aad25a8b6fbd5a2e5788fc44