mmc: sdhci-omap: Fix build if CONFIG_PM_SLEEP is not set

Commit f433e8aa ("mmc: sdhci-omap: Implement PM runtime functions")
combined the use of runtime PM and system suspend functions but left the
ifdef CONFIG_PM_SLEEP in place causing undeclared identifier error for
sdhci_omap_runtime_suspend if CONFIG_PM_SLEEP is not enabled.

Let's fix the error by removing ifdef CONFIG_PM_SLEEP and tagging the
reset of the PM related functions with __maybe_unused.

Let's also remove the forward declaration for sdhci_omap_context_save(),
that was accidentally left from an earlier version and is no longer used.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
[Ulf: Rebased and fixed build error]
Link: https://lore.kernel.org/r/20211021134352.10135-1-tony@atomide.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>