spi: omap2-mcspi: Call pm_runtime_* functions directly

Call the pm_runtime functions directly making room for possible
pm optimisations. Also the runtime functions aren't just about
enabling and disabling of clocks though it does enable clocks also.

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>