[autoboot] Fix incorrect boolean logic

Commit 53d2d9e ("[uri] Generalise tftp_uri() to pxe_uri()") introduced
a regression in which an NFS root path would no longer be treated as
an unsupported root path, causing a boot with an NFS root path to fail
with a "Could not open SAN device" error.

Reported-by: David Evans <dave.evans55@googlemail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

 	 * it may represent an NFS root.
 	 */
 	if ( filename && root_path &&
-	     ( ! ( uri_is_absolute ( root_path ) ||
-		   ( xfer_uri_opener ( root_path->scheme ) == NULL ) ) ) ) {
+	     ( ( ! uri_is_absolute ( root_path ) ) ||
+	       ( xfer_uri_opener ( root_path->scheme ) == NULL ) ) ) {
 		printf ( "Ignoring unsupported root path\n" );
 		uri_put ( root_path );
 		root_path = NULL;