fs/mount/fs_mount.c: Remove a redandunt assignment

Found by clang-check:

mount/fs_mount.c:287:8: warning: Although the value stored to 'ret' is used in
      the enclosing expression, the value is never actually read from 'ret'
      (ret = find_blockdriver(source, mountflags, &drvr_inode)) >= 0)
       ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

fs/mount/fs_mount.c

@@ -284,7 +284,7 @@ int nx_mount(FAR const char *source, FAR const char *target,
 
 #ifdef BDFS_SUPPORT
   if (source != NULL &&
-      (ret = find_blockdriver(source, mountflags, &drvr_inode)) >= 0)
+      find_blockdriver(source, mountflags, &drvr_inode) >= 0)
     {
       /* Find the block based file system */