usb: musb: omap2430: fix kernel panic on reboot

commit b193b41 upstream.

Cancel idle timer in musb_platform_exit.

The idle timer could trigger after clock had been disabled leading to
kernel panic when MUSB_DEVCTL is accessed in musb_do_idle on 2.6.37.

The fault below is no longer triggered on 2.6.38-rc4 (clock is disabled
later, and only if compiled as a module, and the offending memory access
has moved) but the timer should be cancelled nonetheless.