USB: serial: sierra: fix bogus alternate-setting assumption

Interface numbers do not change when enabling alternate settings as
comment and code in this driver suggested.

Remove the confusing comment and redundant retrieval of the interface
number in probe, while simplifying and renaming the interface-number
helper.

Fixes: 4db2299d ("sierra: driver interface blacklisting")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>