mfd: omap-usb-tll: serialize access to TLL device

Get rid of the unnecessary spin_lock_irqsave/restore() as there is
no interrupt handler for this driver. Instead we serialize access
to tll_dev using a global spinlock.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>