Bugfix SPI_TC_Init numspidevices handling

An incorrect assumption was made that SPI_TC_Work could be used during
init (before numspidevices was properly initialized). numspidevices
variable is now properly initialized before attemting to probe (and
still kept at zero until init is run).
Thanks to patrickwtlaw for tracking this thing down!

src/max31855.c

@@ -48,7 +48,6 @@ static int32_t SPI_TC_Work( void ) {
 }
 
 uint32_t SPI_TC_Init( void ) {
-	uint32_t numspidevices = 0;
 	printf("\n%s called",__FUNCTION__);
 	Sched_SetWorkfunc( SPI_TC_WORK, SPI_TC_Work );
 
@@ -60,7 +59,9 @@ uint32_t SPI_TC_Init( void ) {
 	if( SC18IS602B_Init( SPICLK_1843KHZ, SPIMODE_0, SPIORDER_MSBFIRST ) >= 0 ) { // Only continue of we find the I2C to SPI bridge chip
 		printf("\nProbing for MAX31855 devices...");
 
+		numspidevices = MAX_SPI_DEVICES; // Assume all devices are present for SPI_TC_Work
 		SPI_TC_Work(); // Run one iteration to update all data
+		numspidevices = 0; // And reset it afterwards
 
 		for( int i = 0; i < MAX_SPI_DEVICES; i++ ) {
 			if( (spidevreadout[i] == -1 && spiextrareadout[i] == -1) ||