Change SparkFun_APDS9960 to submodule

[mikee47]

Submodule allows easier maintenance and tracking of upstream changes.
Only one function change, addition of getStatusRegister function.

Also patch fixing issue flagged by clang build:

 /home/runner/work/Sming/Sming/Sming/Libraries/SparkFun_APDS9960/SparkFun_APDS9960.cpp:508:32:
warning: result of comparison of constant -1 with expression of type 'uint8_t' (aka 'unsigned char') is always false
[-Wtautological-constant-out-of-range-compare]
                if( bytes_read == -1 ) {
                    ~~~~~~~~~~ ^  ~~