serial: xilinx: uartlite: Fix infinite spin in xlnx_uartlite_fifo_read

The xlnx_uartlite_fifo_read function would spin indefinitely if there
was less data available in the RX FIFO than the size of the passed-in
buffer. This call is supposed to be non-blocking.

Fixed to break out of the loop if there are no more bytes left in the RX
FIFO.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>