spi: gpio: Add local struct spi_bitbang pointer in spi_gpio_probe()

Use a local "struct spi_bitbang *bb" in spi_gpio_probe() for
brevity. No functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Chris Healy <cphealy@gmail.com>
Cc: linux-spi@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>