spi: omap3_spi: Fix speed and mode selection

McSPI IP provides per CS specific speed and mode selection. Therefore it
is possible to apply these settings only after CS is known. But
set_speed and set_mode can be called without bus being claimed, this
would lead driver to set up wrong CS (or previously used CS).

Fix this by apply set_speed and set_mode only if bus is already claimed.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>