windows: Fix WinUSB isochronous transfer regression

Put back missing CHECK_WINUSBX_AVAILABLE macro.

The subapi is generally set in the backend functions themselves, using
the macro CHECK_WINUSBX_AVAILABLE. However, this macro was (likely
inadvertently) removed from winusbx_copy_transfer_data() in commit
9c28ad2 causing the subapi in this function to always be SUB_API_NOTSET.
This results in the error:

  libusb: debug [winusbx_copy_transfer_data] unsupported API call for 'copy_transfer_data' (unrecognized device driver)
References #749
Closes #1095
2 files changed