usb: common: expand documentation for USB functions

This patch adds documentation for two USB functions:
- usb_otg_state_string(), which returns a human-readable name for OTG
  states and
- usb_get_dr_mode_from_string(), which returns the dual role mode for a
  given string.

Signed-off-by: Lucy Mielke <lucymielke@icloud.com>
Link: https://lore.kernel.org/r/6nvegfmo6d5ak4soaf5nyifsaasfts4qlsnnpsd4sgpnikc2jd@amfmgdr5n3bi


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>