IOWin: rework _IOWrite OVERLAPPED/bytes written handling.

CheckDeviceType_Write uses an OVERLAPPED structure to make sure some bytes
were written to the device for a given _IOWrite operation. However, the
OVERLAPPED structure is only used for the BlueSoleil stack, not the MS stack
(which goes through an alternate HID path instead of WriteFile, and does not
have an equivalent async operation).

Make _IOWrite return the number of written bytes instead, since it knows
about the Bluetooth stack being used.