rtlwifi: use SIMPLE_DEV_PM_OPS

rtlwifi only provides pm callbacks for functions covered by pm sleep
and they are also just called if CONFIG_PM_SLEEP is set.

Only add functions rtl_pci_suspend and rtl_pci_resume if
CONFIG_PM_SLEEP is set and use SIMPLE_DEV_PM_OPS instead of
manually filling struct dev_pm_ops.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>