usb: core: use kbasename() instead of open-coded variant

kbasename() helper is dedicated to find a last part of the filename or
pathname. USB core uses open-coded variant of that helper.

Replace some lines of code by kbasename() call.

The current users do not have trailing slash and we are on the safe side to
make such change. I dig a history of the code under question up and found the
patch [1] that brought it along with the same to tty layer. The check for
trailing slash looks like copy'n'paste thing and I consider it as redundant.

[1] http://www.kernel.org/pub/linux/kernel//people/akpm/patches/2.5/2.5.69/2.5.69-mm3/broken-out/linus.patch



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