staging: rts5129: re-use kbasename()

The custom filename function mostly repeats the kernel's kbasename. This patch
simplifies it. The updated filename() will not check for the '\' in the
filenames. It seems redundant in Linux. The __FILE__ macro always defined if we
compile an existing file. Thus, NULL check is not needed there as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>