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

[antevir]

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.

[github-actions]

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[github-actions]

Hi @antevir!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁