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/nvram.c b/env/nvram.c
index ac389ac..85af37d 100644
--- a/env/nvram.c
+++ b/env/nvram.c
@@ -41,7 +41,7 @@
 #endif
 
 #ifdef CONFIG_SYS_NVRAM_ACCESS_ROUTINE
-static uchar env_nvram_get_char(int index)
+static int env_nvram_get_char(int index)
 {
 	uchar c;