usb: musb: Add PM runtime support for MUSB DSPS glue layer

We can now just use PM runtime autoidle support as musb core
keeps things enabled when the devctl session bit is set. And
there's no need for dsps_musb_try_idle() so let's just remove
it.

Note that as cppi41 dma is clocked by musb, this only makes
PM work for dsps glue layer if CONFIG_MUSB_PIO_ONLY=y and
cppi41.ko is unloaded. This will get fixed when cppi41.c has
PM runtime implemented.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>