mtd: cmdlineparts: Use 64-bit format when printing a debug message.

Commit 69423d99

 ("[MTD] update internal 
API to support 64-bit device size") has changed some structure values
to 64-bit and has not updated this debug message, since it's not built 
by default.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>