spi_s3c24xx: fix transfer setup code

Since the changes to the bitbang driver, there is the possibility we will
be called with either the speed_hz or bpw values zero.  We take these to
mean that the default values (8 bits per word, or maximum bus speed).

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>