stm32mp1: board update command stboard on misc_read result

Update management of misc_read, which now return length of data
after the commit 8729b1ae2c ("misc: Update read() and write()
methods to return bytes xfered")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

board/st/stm32mp1/cmd_stboard.c

@@ -60,7 +60,7 @@ static int do_stboard(cmd_tbl_t *cmdtp, int flag, int argc,
 	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;
 	}