serial: meson: handle RX errors

This checks and handles RX errors on the Amlogic UART controller
after experiencing errors on the Khadas VIM3 & VIM3L when UART AO A
lines are not connected.

When the RX line is not connected, the first byte is erroneous and breaks
the U-Boot autoboot, breaking automatic boot.

This checks and drops any erroneous RX byte on pending and getc callbacks
to avoid returning true to pending when an error byte is in the FIFO.

Fixes: bfcef28a ("arm: add initial support for Amlogic Meson and ODROID-C2")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Guillaume La Roque <glaroque@baylibre.com>