OpenSBI: Fixed ecall return

Helps to actually return the right values...

Ecall was returning the passed arguments a0 and a1 instead of the actual
return values from the registers(r_a0 and r_a1)

Shit explanation just read the diff, its one line

kernel/drivers/opensbi/opensbi.cpp

@@ -25,7 +25,7 @@ namespace opensbi {
                 "r"(r_a7), "r"(r_a6),  // Outputs
                 "r"(r_a0), "r"(r_a1), "r"(r_a2),
                 "r"(r_a3), "r"(r_a4), "r"(r_a5));
-        return {.error = a0, .value = a1};
+        return {.error = r_a0, .value = r_a1};
     }
 
     SbiRet ecall(