spi_s3c24xx.c: Fix build

Commit a836f585 removes the shutdown
member of the bitbang structure, breaking the build of spi_s3c24xx.c.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.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>