USB: gr_udc: remove dentry storage for debugfs file

There is no need to store the dentry pointer for a debugfs file that we
only use to remove it when the device goes away.  debugfs can do the
lookup for us instead, saving us some trouble, and making things smaller
overall.

Cc: Felipe Balbi <balbi@kernel.org>
Link: https://lore.kernel.org/r/20210527101426.3283214-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>