spi: davinci_spi: Driver cleanup

Arrange driver code in more readable format[1] for easy accessing
and readable.
[1] http://patchwork.ozlabs.org/patch/265683/



Signed-off-by: Jagan Teki <jteki@openedev.com>
Cc: Rex Chang <rchang@ti.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>