mwl8k: remove useless pci shutdown callback and stray debugging

This patch removes a left over debugging print present in the pci
shutdown callback, since this callback does not do anything useful, get
rid of it entirely.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>