drivers: serial: emul: Make fifo_read() respect size

uart_emul_fifo_read() didn't regard the size arg, hence if the RX ring
buffer contained more data than the size of the input rx_data buffer
size, this could result in buffer overflow. This becomes more obvious
when configuring device tree property latch-buffer-size > 1.

Signed-off-by: Andreas Anderberg <andreas.anderberg@u-blox.com>