staging: comedi: amplc_pc236: Change return type of pc236_find_pci()

pc236_find_pci() finds a supported PCI device, returning 0 on success
or -EIO on failure and returning the pointer to the PCI device via a
struct pci_dev ** parameter.  Change it to return the struct pci_dev *
on success or NULL on failure.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>