brcmsmac: Fix possible NULL pointer dereference in _dma_ctrlflags()

There's a debug message to warn if this function is passed a NULL
pointer, but in order to print the message we have to dereference the
pointer. Obviously this isn't a good idea, so remove the message.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>