mtd: cmdlinepart: Replace `dbg()` macro with `pr_debug()`

This macro was left over from the dawn of Git history.
For some reason it was defined in such a way that
you needed double brackets around it. Replace it with
the now-standard `pr_debug()`.

Signed-off-by: Csókás, Bence <csokas.bence@prolan.hu>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240625093148.3579660-1-csokas.bence@prolan.hu