spi_s3c24xx: use resource_size() to get resource size

Change the use of (res->end - res->start) to use resource_size() to
get the size of the resource.

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>