ni_670x: Fix possible invalid pointer dereference in ni_670x_detach()

ni_670x: Fix possible invalid pointer dereference in ni_670x_detach()

In ni_670x_detach(), dev->subdevices will be NULL if the call to
alloc_subdevices() failed in ni_670x_attach(), so don't dereference it
until the pointer has been checked.

Spotted by H Hartley Sweeten in the "staging" sources, but used a
different check due to difference in behavior of alloc_subdevices() on
error between the two versions.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>