ath5k: depend on CONFIG_PM_SLEEP for suspend/resume functions

When building a kernel with CONFIG_PM=y but neither suspend nor
hibernate support, the compiler complains about the static functions
ath5k_pci_suspend() and ath5k_pci_resume() not being used:

drivers/net/wireless/ath/ath5k/base.c:713:12: warning: ‘ath5k_pci_suspend’ defined but not used
drivers/net/wireless/ath/ath5k/base.c:722:12: warning: ‘ath5k_pci_resume’ defined but not used

Depending on CONFIG_PM_SLEEP rather than CONFIG_PM fixes the issue.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>