The ad9371 call set_master_clock_rate() can take a while depending on
the rate change, so make it asynchronous in order not to lock out the
reclaimer loop.
Added set_sync_source method to set both the time and clock sources
without forcing a re-init twice. Modified the existing set_time_source
and set_clock_source methods to call into set_sync_source.
When a device is re-initialized without any changes (e.g.,
master_clock_rate, ref_clock_freq) then we can skip the initialization
sequence and move on. This shaves a significant amount of time from the
init sequence.
Fast re-init can be overridden by providing the `force_reinit=1` device
arg.
