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>