cmd: bootefi: Fix fdt_size variable type in efi_carve_out_dt_rsv()

Variable fdt_size should be of type 'fdt_size_t', not 'fdt_addr_t'.

Fixes 0d7c2913: ("cmd: bootefi: Honor the address & size cells properties correctly")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>