usb: f_sdp: handle the spl load function failure

usb: f_sdp: handle the spl load function failure

Current implementation does not check the return value of spl load
function. If the spl load is failed, SPL may meet crash due to
spl_image variable is not initialized. Add the failure check,
so SPL can print and stop with error.

Signed-off-by: Ye Li <ye.li@nxp.com>
Fixes: 2c72ead73874 ("usb: gadget: f_sdp: Allow SPL to load and boot FIT via SDP")
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://patch.msgid.link/20260522072705.1156220-1-ye.li@nxp.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>