LLC: Don't blindly assume all LLC frames have data payload

In reality, only UI, I, SABM, UA and XID frames carry payload.  All
other frames will have llhp.data == NULL.

Let's therefore not do any msgb adjustments unless we actually know
there is a user payload field.

Change-Id: I51bbd0f2c618d477a037af343ff41de1c8a5a3ae
Closes: OS#3952