x86: decent declarations in perf_counter.c

Impact: cleanup

making decent declrations for struct pmc_x86_ops and
fix checkpatch error:
 ERROR: Macros with complex values should be enclosed in parenthesis

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>