usb: musb: kill global and static for multi instance

Moved global variable "musb_debugfs_root" and static variable
"old_state" to 'struct musb' to help support multi instance of
musb controller as present on AM335x platform.

Also removed the global variable "orig_dma_mask" and filled the
dev->dma_mask with parent device's dma_mask.

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Santhapuri, Damodar <damodar.santhapuri@ti.com>
Signed-off-by: Ravi Babu <ravibabu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>