It is simply a matter of bits. In the example shown, the hexpairs are disassembled using 64bit assembly, and you probably want to change it to a 32bit assembly. Just tell radare that you are working with 32bits and it will do the job for you:
[0x00000000]> pad 8d 15 c8 90 04 08
lea edx, [rip + 0x80490c8]
[0x00000000]> e asm.bits =32
[0x00000000]> pad 8d 15 c8 90 04 08
lea edx, [0x80490c8]
Unlike 32bits modes instructions that were taken as absolute addresses (use 32-bit immediate offset addressing), the 64bits modes (a.k.a long-mode) are usually using 32-bit offset from the current RIP, not from 0x00000000 like before. That means that you don't have to know the absolute address of something you want to reference, you only need to know how far away it is from the currently executing instruction.
There are very few addressing modes which use a full 64bit absolute address. Most addressing modes are 32bit offsets relative to one of the 64bit registers (usually RIP).