USB: gadget: pxa27x: Re-use DEFINE_SHOW_ATTRIBUTE() macro

...instead of open coding file operations followed by custom ->open()
callbacks per each attribute.

Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>