ARM: perf: remove useless return and check of idx in counter handling

Idx sanity check was once implemented separately in these counter
handling functions and then return value was treated as a judgement.
	armv7_pmnc_select_counter()
	armv7_pmnc_enable_counter()
	armv7_pmnc_disable_counter()
	armv7_pmnc_enable_intens()
	armv7_pmnc_disable_intens()
But we do not need to do this now, as idx validation check was moved
out all these functions by commit 7279adbd(ARM: perf: check ARMv7
counter validity on a per-pmu basis).
Let's remove the useless return of idx from these functions.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: chai wen <chaiw.fnst@cn.fujitsu.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>