fs: exfat: Inhibit "impossible" print on write to bogus file

Write into a bogus file, like '/.', triggers an "impossible"
print from the exfat core code. That should not be printed
in U-Boot, because U-Boot prints its own error message sooner.
Inhibit this error message.

The following command triggers the bogus print:
"
=> save host 0:0 1000008 /. 0x10
"

Fixes: b86a651b64 ("fs: exfat: Add U-Boot porting layer")
Signed-off-by: Marek Vasut <marex@denx.de>

fs/exfat/lookup.c

@@ -218,8 +218,9 @@ int exfat_split(struct exfat* ef, struct exfat_node** parent,
 		exfat_put_node(ef, *parent);
 		*parent = *node;
 	}
+#ifndef __UBOOT__
 	exfat_bug("impossible");
-#ifdef __UBOOT__
+#else
 	return 0;
 #endif
 }