musb_core: don't prevent disabling clock on driver unload

Resetting 'musb->clock' to NULL in musb_shutdown() prevents musb_platform_exit()
from properly disabling the clock when unloading the driver -- don't do it.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>