env: Adjust the get_char() method to return an int

In principle this can fail, e.g. if the index is out of range. Adjust the
driver signature to allow returning an error code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/env/dataflash.c b/env/dataflash.c
index 28dcf6f..afa08f8 100644
--- a/env/dataflash.c
+++ b/env/dataflash.c
@@ -14,7 +14,7 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-static unsigned char env_dataflash_get_char(int index)
+static int env_dataflash_get_char(int index)
 {
 	uchar c;