bpf: neg instruction does not accept an immediate

The BPF virtual machine does not support neg nor neg32 instructions with
an immediate.

The erroneous instructions were removed from binutils:
https://sourceware.org/pipermail/binutils/2023-August/129135.html

Change the define_insn so that an immediate cannot be accepted.

From testing, a neg-immediate was probably never chosen over a
mov-immediate anyway.

gcc/

	* config/bpf/bpf.md (neg): Second operand must be a register.