Given a configuration, find the alternate setting for the given interface.
struct usb_host_interface * usb_find_alt_setting(struct usb_host_config * config, unsigned int iface_num, unsigned int alt_num);
config
the configuration to search (not necessarily the current config).
iface_num
interface number to search in
alt_num
alternate interface setting number to search for.
Search the configuration's interface cache for the given alt setting.
The alternate setting, if found. NULL otherwise.