Fix warning unused variable in readGesture and bit shifting in read32

[dodo5522]

• Arduino board: Original board with 328PB
• Arduino IDE version (found in Arduino -> About Arduino menu): 1.8.12
• List the steps to reproduce the problem below (if possible attach a sketch or copy the sketch code in too): build my sample ino

I got the following warnings.

/Users/takashi/Documents/Arduino/libraries/Adafruit_APDS9960-1.1.3/Adafruit_APDS9960.cpp: In member function 'uint8_t Adafruit_APDS9960::readGesture()':
/Users/takashi/Documents/Arduino/libraries/Adafruit_APDS9960-1.1.3/Adafruit_APDS9960.cpp:394:19: warning: variable 'bytesRead' set but not used [-Wunused-but-set-variable]
   uint8_t toRead, bytesRead;
                   ^~~~~~~~~
/Users/takashi/Documents/Arduino/libraries/Adafruit_APDS9960-1.1.3/Adafruit_APDS9960.cpp: In member function 'uint32_t Adafruit_APDS9960::read32(uint8_t)':
/Users/takashi/Documents/Arduino/libraries/Adafruit_APDS9960-1.1.3/Adafruit_APDS9960.cpp:643:21: warning: left shift count >= width of type [-Wshift-count-overflow]
   return (ret[0] << 24) | (ret[1] << 16) | (ret[2] << 8) | ret[3];
                     ^~
/Users/takashi/Documents/Arduino/libraries/Adafruit_APDS9960-1.1.3/Adafruit_APDS9960.cpp:643:38: warning: left shift count >= width of type [-Wshift-count-overflow]
   return (ret[0] << 24) | (ret[1] << 16) | (ret[2] << 8) | ret[3];
                                      ^~

There is comment of bytesRead is unused but produces sideffects needed for readGesture to work but it seems OK with only reading APDS9960_GFIFO_U register and bytesRead variable itself can be removed.

And I cast uint32_t before bit shifting.

[dodo5522]

@ladyada Thanks!