kexec/dt-ops.c: Fix '/chosen' v/s 'chosen' node being passed to fdt helper functions

This patch fixes the incorrect 'chosen' node name being passed to
various fdt helper functions inside 'kexec/dt-ops.c'

As we can see from the Linux kernel usage inside
'drivers/firmware/efi/libstub/fdt.c', we pass '/chosen' node names to
fdt helper function like 'fdt_path_offset()' whereas 'chosen' to the
rest of the fdt helper functions like 'fdt_subnode_offset()'.

We need to replicate the same in 'kexec-tools' to fix issues being
reported when we use --dtb option while invoking 'kexec'.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>