spl: don't assume NVMe partition 1 exists

There is no requirement that a partition 1 exists in a partition table.
We should not try to retrieve information about it.

We should not even try reading with partition number
CONFIG_SYS_NVME_BOOT_PARTITION here as this is done in the fs_set_blk_dev()
call anyway.

Fixes: 8ce6a2e1 ("spl: blk: Support loading images from fs")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>