Add usbg_udc structure

Dear Matt,

This series adds usbg_udc structure and fixes some typos introduced
in previous patches.

Before this series UDC didn't have its own structure. Gadget enable
took const char * as parameter. Such approach is inconsisten with
rest of API.

This series introduce structure usbg_udc as abstraction of UDC.
This makes API more consistent and some functions (like enable
gadget) are now shorter and easier to read. Moreover usage of
such structure allows to implement usbg_get_udc_gadget() which
provides possibility to check which gadget is enabled on selected UDC.

In addition this series fix a lot of typos introduced in previous
patches. Now doxygen generates documentation without errors.