x86/hyperv: Fix unused variable 'hi' warning in hv_apic_read

Fixes the following W=1 kernel build warning(s):
arch/x86/hyperv/hv_apic.c:58:15: warning: variable ‘hi’ set but not used [-Wunused-but-set-variable]

Compiled with CONFIG_HYPERV enabled:
make allmodconfig ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu-
make W=1 arch/x86/hyperv/hv_apic.o ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu-

HV_X64_MSR_EOI occupies bit 31:0 and HV_X64_MSR_TPR occupies bit 7:0,
which means the higher 32 bits are not really used. Cast the variable hi
to void to silence this warning.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xu Yihang <xuyihang@huawei.com>
Link: https://lore.kernel.org/r/20210323025013.191533-1-xuyihang@huawei.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

arch/x86/hyperv/hv_apic.c

@@ -60,9 +60,11 @@ static u32 hv_apic_read(u32 reg)
 	switch (reg) {
 	case APIC_EOI:
 		rdmsr(HV_X64_MSR_EOI, reg_val, hi);
+		(void)hi;
 		return reg_val;
 	case APIC_TASKPRI:
 		rdmsr(HV_X64_MSR_TPR, reg_val, hi);
+		(void)hi;
 		return reg_val;
 
 	default: