gnss: sirf: add support for configurations without wakeup signal

Some Wi2Wi devices do not have a wakeup output, so device state can
only be indirectly detected by looking whether there is communication
over the serial lines.
This approach requires a report cycle set to a value less than 2 seconds
to be reliable.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
[ johan: simplify sirf_set_active(), style changes ]
Signed-off-by: Johan Hovold <johan@kernel.org>