HID: intel-ish-hid: Add interface function for PCI device pointer

Instead of directly accessing PCI device poitner via struct ishtp_cl,
create interface function for same. This is required for DMA transfer.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>