call callback only if set
call callback only if set

thomaschaaf/node-ftdi,KABA-CCEAC/node-ftdi,thomaschaaf/node-ftdi,thomaschaaf/node-ftdi,thomaschaaf/node-ftdi,KABA-CCEAC/node-ftdi,KABA-CCEAC/node-ftdi,thomaschaaf/node-ftdi,KABA-CCEAC/node-ftdi,KABA-CCEAC/node-ftdi