bootstd: Fix incorrect struct name in bootmeth_setup_iter_order()

bootstd: Fix incorrect struct name in bootmeth_setup_iter_order()

There is no such thing as struct bootmeth, it's probably a typo. This
issue doesn't affect the execution as it's a pointer, and pointer sizes
are the same for all data types. But it can be confusing, so make it
struct udevice, as it should be.

Fixes: a950d31abe98 ("bootstd: Add the bootmeth uclass and helpers")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>