arm64: fix return code check when changing emulation handler

update_insn_emulation_mode() returns 0 on success, so we should be
treating any non-zero values as failure, rather than the other way
around. Otherwise, writes to the sysctl file controlling the emulation
are ignored and immediately rolled back.

Reported-by: Gene Hackmann <ghackmann@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>