MLK-11689: dma: imx-sdma: correct channel context restore

Before, checking SDMA_H_C0PTR register to know whether sdma controller turned
off in DSM, if yes restore channel context back. Unfortunatly, this checking
is wrong, because SDMA_H_C0PTR has been initialized as non-zero value in
sdma_resume, which means channel context will never be restored back if mega/
fast off on i.mx6sx or i.mx7d. Using 'suspend_off' flag to keep this 'restore
needed' requirement.

Signed-off-by: Robin Gong <b38343@freescale.com>