Fixed the iManufacturer and iProduct HID string descriptor functions

[gigaj0ule]

iManucaturer and iProduct were not properly written for HID devices, as the setup.wlength struct element did not represent the real length of the string descriptor.

This caused HID driver errors on Windows AMD_64.

String descriptors are supposed to be in unicode, so 2*sizeof(string) gives the proper length.

[facchinm]

LGTM, testing it!

edit: the second parameter of sendStringDescriptor is maxlen, not the actual length; it actually sends the right length of the string (when "Unicoded") IF the string is \0 terminated. Probably the HID problem arises from a missing termination that makes the sting longer than expected.
Could you post the problematic sketch/library in #274 ? Thanks! Closing this one in the meantime, will reopen if needed.