add flags set method for tansfer

[wang0618]

This PR adds a setFlags() method to USBTransfer to set the libusb_transfer.flags

[vpelletier]

My apologies, I completely missed the bug report you opened as well as this merge request.

I agree that flags need to be exposed.

This said, of the currently-defined 4 flags, two would cause crashes, so the code must prevent the user from setting those. This would probably best be done by only allowing the known-safe flags, raising if the caller tries to set anything else.

I also think the currently-set flags should be exposed, probably by adding a getFlags method. I wonder what return type would be best.

Could you implement such changes and update this merge request ?