oprofile, ARM: Use oprofile_arch_exit() to cleanup on failure

There is duplicate cleanup code in the init and exit functions. Now,
oprofile_arch_exit() is also used if oprofile_arch_init() fails.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>