spi: davinci: shorten variable names

Shorten names of local variables and structure members where
possible.

Local variables:

* 'davinci_spi' is being renamed 'dspi'
* 'davinci_spi_dma' is being renamed 'dma'

Structure members:

* 'dma_{tx|rx}_channel' is being renamed '{tx|rx}_channel'
  since the structure containing them is already called
  'davinci_spi_dma'

* 'davinci_spi_dma' in 'davinci_spi' structure is being
  renamed 'dma'
Tested-By: Brian Niebuhr <bniebuhr@efjohnson.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>