x86/bugs: Fix return type of spectre_bhi_state()

The definition of spectre_bhi_state() incorrectly returns a const char
* const. This causes the a compiler warning when building with W=1:

 warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 2812 | static const char * const spectre_bhi_state(void)

Remove the const qualifier from the pointer.

Fixes: ec9404e ("x86/bhi: Add BHI mitigation knob")
Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240409230806.1545822-1-daniel.sneddon@linux.intel.com

arch/x86/kernel/cpu/bugs.c

@@ -2809,7 +2809,7 @@ static char *pbrsb_eibrs_state(void)
 	}
 }
 
-static const char * const spectre_bhi_state(void)
+static const char *spectre_bhi_state(void)
 {
 	if (!boot_cpu_has_bug(X86_BUG_BHI))
 		return "; BHI: Not affected";