dmaengine: Fix doc strings to satisfy validation script

dmaengine: Fix doc strings to satisfy validation script

The validation kernel doc script complains about undescribed
function parameters

.../dmaengine.c:155: warning: Function parameter or member 'dev' not descr ibed in 'dev_to_dma_chan'
.../dmaengine.c:251: warning: cannot understand function prototype: 'dma_cap_mask_t dma_cap_mask_all; '
.../dmaengine.c:257: warning: cannot understand function prototype: 'struct dma_chan_tbl_ent '
.../dmaengine.c:264: warning: cannot understand function prototype: 'struct dma_chan_tbl_ent __percpu *channel_table[DMA_TX_TYPE_END]; '
.../dmaengine.c:304: warning: Function parameter or member 'chan' not described in 'dma_chan_is_local'
.../dmaengine.c:304: warning: Function parameter or member 'cpu' not described in 'dma_chan_is_local'
.../dmaengine.c:414: warning: Function parameter or member 'chan' not described in 'balance_ref_count'
.../dmaengine.c:447: warning: Function parameter or member 'chan' not described in 'dma_chan_get'
.../dmaengine.c:494: warning: Function parameter or member 'chan' not described in 'dma_chan_put'

Add descriptions to the function parameters and in some cases update
existing text as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200429122151.50989-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>