Refactor PORT_MAX_RFC_PORTS

The Bluetooth RFCOMM Spec dictates that a Server Channel Number (SCN)
must be in the range of [1, 30].
- Rename PORT_MAX_RFC_PORTS to RFCOMM_MAX_SCN
- Change the value from 31 to 30 and refactor code such that it
  mechanically functions the same

Bug: 299158235
Test: atest BtmAllocateSCNTest
Change-Id: If27d19bd675c7fb13a5b2c047240d2f10808ee64