lib:platform: Fix sbi_getc return type.

As per the current SBI specification, sbi_getc should return
an int instead of char.

In case of FIFO is empty, return -1 as per the specification.

Reported-by: Sergi Granell <xerpi.g.12@gmail.com>
Suggested-by:Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
Signed-off-by: Atish Patra <atish.patra@wdc.com>

platform/kendryte/k210/platform.c

@@ -31,7 +31,7 @@ static void k210_console_putc(char c)
 	uarths_putc(c);
 }
 
-static char k210_console_getc(void)
+static int k210_console_getc(void)
 {
 	return uarths_getc();
 }