spi: fix spi_s3c24xx_gpio device handle lookup

The spidev_to_sg() call in spi_s3c24xx_gpio.c was using the wrong method
to convert the spi device into the private data for the driver.  Fix this
by using spi_master_get_devdata.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>