arm64: Correct ftrace calls to aarch64_insn_gen_branch_imm()

The aarch64_insn_gen_branch_imm() function takes an enum as the last
argument rather than a bool. It happens to work because
AARCH64_INSN_BRANCH_LINK matches 'true' but better to use the actual
type.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>