lib: Return ENOTSUPP incase of invalid SBI function ID

OpenSBI should show error trace only if any valid SBI function
does not perform as expected.

However, OpenSBI should show notify the caller with a negative
error if given SBI function ID is not valid.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

lib/sbi_ecall.c

@@ -86,6 +86,8 @@ int sbi_ecall_handler(u32 hartid, ulong mcause,
 		ret = 0;
 		break;
 	default:
+		regs->a0 = SBI_ENOTSUPP;
+		ret = 0;
 		break;
 	};