mmc: moxart: fix probe logic

Jonas Jensen wanted to submit a patch for these, but apparently
forgot about it. I stumbled over this symptom first:

drivers/built-in.o: In function `moxart_probe':
:(.text+0x2af128): undefined reference to `of_dma_request_slave_channel'

This is because of_dma_request_slave_channel is an internal helper
and not exported to loadable module. I'm changing the driver to
use dma_request_slave_channel_reason() instead.

Further problems from inspection:

* The remove function must not call kfree on the host pointer,
  because it is allocated together with the mmc_host.

* The clock is never released

* The dma_cap_mask_t is completely unused and can be removed

* deferred probing does not work if the dma driver is loaded
  after the mmc driver.

This patch should fix all of the above.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>