sd: don't mess with SD_MINORS for CONFIG_DEBUG_BLOCK_EXT_DEVT

No need to give up the original sd minor even with this option,
and if we did we'd also need to fix the number of minors for
this configuration to actually work.

Fixes: 7c3f828b

 ("block: refactor device number setup in __device_add_disk")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>