usb: musb: save dynfifo in musb struct

Save dynamic FIFO read only information for later uses during
musb_save/restore_context functions.

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>