pico: fix tca9555 gpio read initial value

Now we don't end up with all buttons pressed if the i2c gets disconnected

32blit-pico/input/tca9555.cpp

diff --git a/32blit-pico/input/tca9555.cpp b/32blit-pico/input/tca9555.cpp
index 35302ab..43349ef 100644
--- a/32blit-pico/input/tca9555.cpp
+++ b/32blit-pico/input/tca9555.cpp
@@ -42,7 +42,7 @@
   if(!tca9555_found)
     return;
 
-  uint16_t gpio = 0;
+  uint16_t gpio = 0xFFFF;
 
   i2c_read_blocking(TCA9555_I2C, TCA9555_ADDR, (uint8_t *)&gpio, 2, false);