USB: mon: no need to check return value of debugfs_create functions

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.

We do need to save the dentries for these files, so keep them around,
but no need to check if they are "valid" or not, as the code works just
as well either way.

Cc: Pete Zaitcev <zaitcev@kotori.zaitcev.us>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Fredrik Noring <noring@nocrew.org>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>