perf/x86/intel/p4: Fix unused variable warning in p4_pmu_init()

Build the kernel with make W=1 generates the following warning:

  arch/x86/events/intel/p4.c: In function ‘p4_pmu_init’:
  arch/x86/events/intel/p4.c:1370:27: error: variable ‘high’ set but not used [-Werror=unused-but-set-variable]
   1370 |         unsigned int low, high;
        |                           ^~~~

This happens because, although both variables are declared and
initialized by rdmsr, only `low` is used in the subsequent if statement.

This patch uses the rdmsrq() macro instead of the rdmsr() macro.
The rdmsrq() macro avoids the use of high and low variables
because it reads the msr value in a single u64 variable.
Also, replace (1 << 7) with the proper macro.

Running `make W=1` again resolves the error.
I was unable to test the patch because
i do not have the hardware.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Aldo Conte <aldocontelk@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260320112302.281549-1-aldocontelk@gmail.com