AP_Compass: workaround hardware bug in IST8310 whoami

the WAI (whoami) register is writeable. Not only is it writeable, but
the written value is persistent across a power cycle. You have to
remove power for about 30s for it to finally go back to the right
default value of 0x10

this makes using WAI as a test for finding a IST8310 problematic. The
best we can do is send a soft reset which will reset it to default for
us to then check

libraries/AP_Compass/AP_Compass_IST8310.cpp

@@ -113,6 +113,22 @@ bool AP_Compass_IST8310::init()
     // high retries for init
     _dev->set_retries(10);
 
+    /*
+      unfortunately the IST8310 employs the novel concept of a
+      writeable WHOAMI register. The register can become corrupt due
+      to bus noise, and what is worse it persists the corruption even
+      across a power cycle. If you power it off for 30s or more then
+      it will reset to the default of 0x10, but for less than that the
+      value of WAI is unreliable.
+
+      To avoid this issue we do a reset of the device before we probe
+      the WAI register. This is nasty as we don't yet know we've found
+      a real IST8310, but it is the best we can do given the bad
+      hardware design of the sensor
+     */
+    _dev->write_register(CNTL2_REG, CNTL2_VAL_SRST);
+    hal.scheduler->delay(10);
+
     uint8_t whoami;
     if (!_dev->read_registers(WAI_REG, &whoami, 1) ||
         whoami != DEVICE_ID) {