usb: musb: Fix external abort in musb_remove on omap2430

This fixes an oops on unbind / module unload (on the musb omap2430
platform).

musb_remove function now calls musb_platform_exit before disabling
runtime pm.

Signed-off-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>