usb: musb: drop superfluous pm_runtime calls around musb_shutdown

Since commit 4f9edd2d7e8d "usb: musb: Fix the crash issue during reboot"
musb_shutdown() does pm_runtime_get_sync/pm_runtime_put by itself, so
this no longer needs to be done by the caller. Also, musb_exit_debugfs()
doesn't access the device, so just drop those runtime_pm calls.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>