lib: serial: fix RX path in litex-uart

lib: serial: fix RX path in litex-uart

When used to read characters from the terminal (e.g., when the SBI
console is used via ecall from linux with `console=hvc0`), we must
acknowledge receipt of each character to "pop" it off the LiteUART
hardware queue, and allow the next character to be made available.

Fixes: 52af6e4b ("lib: utils: Add LiteX UART support")
Suggested-by: Dolu1990 <charles.papon.90@gmail.com>
Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>