floppy: add FD_AUTODETECT_SIZE define for struct floppy_drive_params

Use FD_AUTODETECT_SIZE for autodetect buffer size in struct
floppy_drive_params instead of a magic number.

Link: https://lore.kernel.org/r/20200501134416.72248-3-efremov@linux.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Denis Efremov <efremov@linux.com>

include/uapi/linux/fd.h

@@ -172,7 +172,10 @@ struct floppy_drive_params {
  * used in succession to try to read the disk. If the FDC cannot lock onto
  * the disk, the next format is tried. This uses the variable 'probing'.
  */
-	short autodetect[8];		/* autodetected formats */
+
+#define FD_AUTODETECT_SIZE 8
+
+	short autodetect[FD_AUTODETECT_SIZE]; /* autodetected formats */
 	
 	int checkfreq; /* how often should the drive be checked for disk 
 			* changes */