USB: musb: Re-use DEFINE_SHOW_ATTRIBUTE() macro

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

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>