mmc: core: check PM_SLEEP for mmc_bus_suspend/resume callbacks

If PM_SLEEP is not enabled, mmc.c will give warnning since mmc_bus_suspend/
mmc_bus_resume functions are defined but not used. This patch can fix this
warnning.

Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>