USB: musb: clean up debugfs file and directory creation

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Because of this, lots of init functions do not need to have return
values, so this cleans up a lot of unused error handling code that never
could have triggered in the past.

Cc: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>