spi: omap2-mcspi: Prevent duplicate gpio_request

Occasionally the setup function will be called multiple times. Only request
the gpio the first time otherwise -EBUSY will occur on subsequent calls to
setup.

Reported-by: Joseph Bell <joe@iachieved.it>

Signed-off-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Mark Brown <broonie@kernel.org>