usb: musb: dsps: compile suspend/resume only with PM_SLEEP

Depending on PM is not enough, because only PM_RUNTIME could be
selected. Fixes:

drivers/usb/musb/musb_dsps.c:703:12: warning: 'dsps_suspend' defined but not used [-Wunused-function]
drivers/usb/musb/musb_dsps.c:721:12: warning: 'dsps_resume' defined but not used [-Wunused-function]

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>