rtw88: fix multiple definition of rtw_pm_ops

'const struct dev_pm_ops rtw_pm_ops' is declared by pci.c, and it should be
declare as 'extern' in pci.h. Without 'extern' causes every file including
pci.h has an individual instance of rtw_pm_ops but not reference to the one
declared in pci.c

If kernel config, like test robot, doesn't build driver as module, it leads
multiple definition.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 2e86ef41 ("rtw88: pci: Add prototypes for .probe, .remove and .shutdown")
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201208013746.11065-1-pkshih@realtek.com