spi: gpio: Use devm_spi_register_master()

Replace spi_bitbang_start() with a combination of spi_bitbang_init()
and devm_spi_register_master() and drop all of the explicit
cleanup-related code that's no longer necessary.

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>