ARM: perf: simplify __hw_perf_event_init err handling

Currently __hw_perf_event_init has an err variable that's ignored right
until the end, where it's initialised, conditionally set, and then used
as a boolean flag deciding whether to return another error code.

This patch removes the err variable and simplifies the associated error
handling logic.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>