LoongArch: Fix definition of ftrace_regs_set_instruction_pointer()

LoongArch: Fix definition of ftrace_regs_set_instruction_pointer()

The current definition of ftrace_regs_set_instruction_pointer() is not
correct. Obviously, this function is used to set instruction pointer but
not return value, so it should call instruction_pointer_set() instead of
regs_set_return_value().

There is no side effect by now because it is only used for kernel live-
patching which is not supported, so fix it to avoid failure when testing
livepatch in the future.

Fixes: 6fbff14a6382 ("LoongArch: ftrace: Abstract DYNAMIC_FTRACE_WITH_ARGS accesses")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>