tty: hvc: udbg_hvc: retry putc on -EAGAIN

hvterm_raw_put_chars() calls hvc_put_chars(), which may return -EAGAIN
when the underlying hcall returns a "busy" status, but udbg_hvc_putc()
doesn't handle this. When using xmon on a PowerVM guest, this can
result in incomplete or garbled output when printing relatively large
amounts of data quickly, such as when dumping the kernel log buffer.

Call again on -EAGAIN.
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210514214422.3019105-1-nathanl@linux.ibm.com

drivers/tty/hvc/hvc_vio.c

@@ -249,7 +249,7 @@ static void udbg_hvc_putc(char c)
 			count = hvterm_hvsi_put_chars(0, &c, 1);
 			break;
 		}
-	} while(count == 0);
+	} while (count == 0 || count == -EAGAIN);
 }
 
 static int udbg_hvc_getc_poll(void)