Correct FPB revision checks
Correct FPB revision checks

FPB revision was being stored in `fp_rev` but often tested from
`fpb_rev`.

Combine these two variables.

Spotted while browsing code - by inspection, this appears to have made
`FPB::can_support_address` pessimistic.

mesheven/pyOCD,mbedmicro/pyOCD,flit/pyOCD,mesheven/pyOCD,pyocd/pyOCD,flit/pyOCD,pyocd/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,mesheven/pyOCD