USB: usblcd: drop redundant lcd mutex

Drop the redundant lcd mutex introduced by commit 925ce689

 ("USB:
autoconvert trivial BKL users to private mutex") which replaced an
earlier BKL use.

The lock serialised calls to open() against other open() and a custom
ioctl() returning the bcdDevice (sic!), but neither is needed.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190926091228.24634-9-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>