fastboot: Fix has-slot command always returning yes for fb_nand

fastboot: Fix has-slot command always returning yes for fb_nand

The issue was a mismatch in return value conventions between functions:
- getvar_get_part_info() expects >= 0 for success
- fb_nand_lookup() returns 0 on success, 1 on failure (from
mtdparts_init and find_dev_and_part)

When partition didn't exist, fb_nand_lookup returned 1, but
fastboot_nand_get_part_info passed it directly to getvar_get_part_info,
which treated 1 >= 0 as success, causing has-slot to always return yes.

Fix by converting positive return values to -ENOENT in
fastboot_nand_get_part_info to match the expected error convention.

Signed-off-by: Chance Yang <chance.yang@kneron.us>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://lore.kernel.org/r/20250826-master-v2-1-30b787a2f9fd@kneron.us
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>