bpf: fix zero_extendqidi2 ldx template

Commit 77d0f9ec inadvertently changed
the normal asm dialect instruction template for zero_extendqidi2 from
ldxb to ldxh. Fix that.

gcc/

	* config/bpf/bpf.md (zero_extendqidi2): Correct asm template to
	use ldxb instead of ldxh.