mtd: gen_probe: Use bitmap_zalloc() when applicable

'chip_map' is a bitmap. So use 'bitmap_zalloc()' to simplify code,
improve the semantic and avoid some open-coded arithmetic in allocator
arguments.

Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/a6fe58dffe553a3e79303777d3ba9c60d7613c5b.1637510255.git.christophe.jaillet@wanadoo.fr