staging: comedi: s626: remove unnecessary 'cmd' pointer checks

The local variable 'cmd' is a pointer to the address of a member variable
of a struct. It will always be valid. Remove the unnecessary checks.

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>