fwu: mtd: Fix dfu_alt_info generation for 2 images per bank

Code rewrites the last char of size with adding &. It is visible from
dfu_alt_info print before this patch:

Make dfu_alt_info: 'mtd nor0=bank0 raw 2320000 80000;bank1 raw 27a0000
8000&mtd nor0=bank0 raw 23a0000 4000000;bank1 raw 2820000 4000000'

And after it:
Make dfu_alt_info: 'mtd nor0=bank0 raw 2320000 80000;bank1 raw 27a0000
80000&mtd nor0=bank0 raw 23a0000 4000000;bank1 raw 2820000 4000000'

Size for bank0 and bank1 must be the same because it is the same image.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>