usb-linux: introduce a usb_linux_alt_setting function

The next patch in this series introduces multiple ways to get the
alt setting dependent upon usb_fs_type, it is cleaner to put this
into its own function.

Note that this patch also changes the assumed alt setting in case
of an error getting the alt setting to be 0 (a sane default) rather
then the interface numberwhich makes no sense.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>