fix a bug in mbed.py
fix a bug in mbed.py

mesheven/pyOCD,pyocd/pyOCD,tgarc/pyOCD,bridadan/pyOCD,devanlai/pyOCD,oliviermartin/pyOCD,geky/pyOCD,matthewelse/pyOCD,molejar/pyOCD,pyocd/pyOCD,bridadan/pyOCD,c1728p9/pyOCD,devanlai/pyOCD,mesheven/pyOCD,mesheven/pyOCD,geky/pyDAPLink,tgarc/pyOCD,geky/pyOCD,wjzhang/pyOCD,0xc0170/pyOCD,mbedmicro/pyOCD,flit/pyOCD,flit/pyOCD,mbedmicro/pyOCD,matthewelse/pyOCD,adamgreen/pyOCD,tgarc/pyOCD,adamgreen/pyOCD,c1728p9/pyOCD,NordicSemiconductor/pyOCD,c1728p9/pyOCD,devanlai/pyOCD,oliviermartin/pyOCD,0xc0170/pyOCD,molejar/pyOCD,wjzhang/pyOCD,wjzhang/pyOCD,adamgreen/pyOCD,0xc0170/pyOCD,matthewelse/pyOCD,mbedmicro/pyOCD,molejar/pyOCD,oliviermartin/pyOCD,bridadan/pyOCD,geky/pyOCDgdb