spi: gpio: Drop mflags argument from spi_gpio_request()

The logic of setting mflags in spi_gpio_request() is very simple and
there isn't much benefit in having it in that function. Move all of
that code outside into spi_gpio_probe() in order to simplify things.

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>