stm32mp1: board update command stboard on misc_read result

Update management of misc_read, which now return length of data
after the commit 8729b1ae2cbd ("misc: Update read() and write()
methods to return bytes xfered")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/board/st/stm32mp1/cmd_stboard.c b/board/st/stm32mp1/cmd_stboard.c
index f781c36..04352ae 100644
--- a/board/st/stm32mp1/cmd_stboard.c
+++ b/board/st/stm32mp1/cmd_stboard.c
@@ -60,7 +60,7 @@
 	ret = misc_read(dev, STM32_BSEC_SHADOW(BSEC_OTP_BOARD),
 			&otp, sizeof(otp));
 
-	if (ret) {
+	if (ret < 0) {
 		puts("OTP read error");
 		return CMD_RET_FAILURE;
 	}