cmd: fdt: Fix fdt rsvmem behavior on non-existent index and error message space

cmd: fdt: Fix fdt rsvmem behavior on non-existent index and error message space

In case 'fdt rsvmem delete index' is passed a non-existent index, one
which does not exist in 'fdt rsvmem print', then the following command
triggers a print of help message for 'fdt' command instead of erroring
out:
=> fdt rsvmem delete 1234
This is because the subcommand errornously returns 'err' instead of
CMD_RET_FAILURE, fix it. Furthermore, align the number of spaces past
fdt_del_mem_rsv() and fdt_add_mem_rsv() in error message with the rest
of the code.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>