staging: rts5208: avoid symbolic permissions in rtsx.c

This patch fixes the following checkpatch.pl warning in rtsx.c:
WARNING: Symbolic permissions are not preferred.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>