spi: davinci: setup chip-select timers values only if timer enabled

Setup chip-select timers values only if timer is enabled
(timer_disbled in spi configuration is false).

As a nice side effect, this patch removes code duplicated in
davinci_spi_bufs_pio() and davinci_spi_bufs_dma().

Signed-off-by: Brian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>