If there are no global instances defined, don't provide default &Seri…

[fionn-r]

This is a fix to issue #111 where if HardwareSerial Serial is not defined, Adafruit_BusIO_Register.h will not be able to find Serial and cause compilation to fail.

The change only affects the scope where either NO_GLOBAL_INSTANCES or NO_GLOBAL_SERIAL is defined, which is the case where it was failing to compile anyway. In all other cases, the change has no impact. If a dependent is requiring the default argument, then the dependent will not compile anyway if there is no global serial.