[PATCH] explicitly define PRD_ENTRIES to 256

From: William Lee Irwin III <wli@holomorphy.com>

PRD_ENTRIES is specified to be precisely 256; on platforms where
PAGE_SIZE varies from 4KB the calculation in the current expression
defining it is inaccurate, which may cause crashes. This patch changes
it to the constant literal 256.

include/linux/ide.h

@@ -224,7 +224,7 @@ typedef unsigned char	byte;	/* used everywhere */
  * allowing each to have about 256 entries (8 bytes each) from this.
  */
 #define PRD_BYTES       8
-#define PRD_ENTRIES     (PAGE_SIZE / (2 * PRD_BYTES))
+#define PRD_ENTRIES	256
 
 /*
  * Some more useful definitions