perf_counter: Fix return value from dummy hw_perf_counter_init

Impact: fix oops-causing bug

Currently, if you try to use perf_counters on an architecture that has
no hardware support, and you select an event that doesn't map to any of
the defined software counters, you get an oops rather than an error.
This is because the dummy hw_perf_counter_init returns ERR_PTR(-EINVAL)
but the caller (perf_counter_alloc) only tests for NULL.

This makes the dummy hw_perf_counter_init return NULL instead.

Signed-off-by: Paul Mackerras <paulus@samba.org>