staging: comedi: ni_660x: remove spinlock 'dma_cfg_lock'

This spinlock is only used to protect changes to the private data 'dma_cfg'.

Before calling any function that would change the 'dma_cfg' the spinlock
'mite_channel_lock' is also locked. That spinlock is not unlocked until
after the 'dma_cfg' change.

Remove the redundant spinlock.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>