serial: pl011: Resend the character if FIFO is full in debug uart

pl01x_putc() might return -EAGAIN if there was no space in FIFO. In that
case, high-level caller should wait until there is space and resend the
character.

Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>