Clearing the DRE flag for the transmit interrupt at the end of a
uart.write() also cleared the RXC flag disabling the receive interrupt.
This commit also changes the flag set/clear mechanism in the driver for SPI
as well, even if it did not cause a problem there. But at least it saves a
few bytes of code.
Applies to both SPI and I2C. The underflow caused high baudrate settings
resulting in the lowest possible baudrate. The overflow resulted in
erratic baudrates, not just the lowest possible.
Protect SerCom (UART, SPI, I2C) objects from getting freed by the GC when
they go out of scope without being deinitialized. Otherwise the ISR of a
Sercom may access an invalid data structure.
