LPC55xx: Abort previous Receive() call when reconfiguring
LPC55xx: Abort previous Receive() call when reconfiguring

`uart_set_configuration()` configures the USART driver baudrate and
starts a new `Receive()` call of one byte which calls again the next one
byte `Receive()` from its callback. `uart_set_configuration()` would
normally be called only once, but there's nothing preventing the
USBD_CDC_ACM code to change the baudrate multiple times. Calling
`Receive()` while another call is pending on the same handle returns
an `ARM_DRIVER_ERROR_BUSY` error and fail.

This commit aborts the ongoing transfer (if any) before starting a new
one.

Without this commit the RX side of the USART0 would not work if
`uart_set_configuration()` was called multiple times after connection,
something that's apparently normal with some Linux serial tools. With
this patch the RX side works reliably.

google/DAPLink-port,google/DAPLink-port,google/DAPLink-port,google/DAPLink-port