ARM: OMAP: Fix the use of uninitialized dma_lch_count

'omap_dma_reserve_channels' when used is suppose to be from command.
    so, it alreay has value before 1st call of omap_system_dma_probe.
    and it will never be changed again during running (not from ioctl).

  but 'dma_lch_count' is zero before 1st call of omap_system_dma_probe.
    so it will be failed for omap_dma_reserve_channels, when 1st call.

  so, need use 'd->lch_count' instead of 'dma_lch_count' for judging.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>