board: phytec: imx8m_som_detection: change phytec_imx8m_detect return type

phytec_imx8m_detect returns -1 on error, but the return type is u8
leading to 255 return values. Fix this by changing the return type to
int; there is no reason to keep it as u8 .

Signed-off-by: Yannic Moog <y.moog@phytec.de>
diff --git a/board/phytec/common/imx8m_som_detection.c b/board/phytec/common/imx8m_som_detection.c
index c6c96ed..45f5767 100644
--- a/board/phytec/common/imx8m_som_detection.c
+++ b/board/phytec/common/imx8m_som_detection.c
@@ -23,7 +23,7 @@
  *
  * Returns 0 in case it's a known SoM. Otherwise, returns -1.
  */
-u8 __maybe_unused phytec_imx8m_detect(struct phytec_eeprom_data *data)
+int __maybe_unused phytec_imx8m_detect(struct phytec_eeprom_data *data)
 {
 	char *opt;
 	u8 som;