linux: Fix spi handling with more than one spi device

Reported by @opensource-alt.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/linux/spidev.c

@@ -57,6 +57,7 @@ spi_open(uint32_t bus, uint32_t dev)
     devices[devices_count].bus = bus;
     devices[devices_count].dev = dev;
     devices[devices_count].fd = fd;
+    devices_count++;
     return fd;
 }