staging/easycap: probe: simplify the endpoints tests

we are interested only in isochronous in endpoints
so we can simplify the flow

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>