ipw2x00: use to_pci_dev()

Use to_pci_dev() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Acked-by: Stanislav Yakovlev <stas.yakovlev@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>