fibocom: don't be strict on required drivers

Requiring an explicit set of drivers should only be done when we have
different plugins supporting different types of devices of the same
manufacturer based on the drivers in use (e.g. as in the Sierra case).

When the plugin includes a list of required drivers, the plugin filter
will only assume the plugin can be used if at least one of the
available ports is exposed with one of the drivers in the list.

Because of this, if we start probing some ports before all the
expected ports are reported, we may end up not selecting a given
plugin just because the driver match should have been done in a
not-yet-notified port.

E.g. on system boot:
 * ttyACM0 is notified.
 * min probing time elapses so ttyACM0 probing starts
 * Fibocom plugin is not selected for ttyACM0 as no matched
   driver ("cdc_acm") is given in the the list of drivers of the
   plugin.
 * cdc-wdm0 and wwan0 drivers are notified.
 * MM creates generic MBIM modem with the Fibocom plugin, but without
   XMM features support, because the ttyACM0 port probing was not done
   by the Fibocom plugin.

And if we reset the modem after that:
 * ttyACM0 is notified.
 * cdc-wdm0 and wwan0 drivers are notified.
 * min probing time elapses so probing starts for all ports
 * Fibocom plugin is selected for all port probings as "cdc_mbim" is
   listed in the list of drivers of the plugin.
 * MM creates XMM-capable MBIM modem with the Fibocom plugin.