[mt8167][gpio] Fix MMIO test methods

Test: mt8167 ref board boots.
Tets: runtests -t mtk-gpio-test
Change-Id: I8c18380558503040fa84956b73349face7694076

system/dev/gpio/mt-8167/mt8167-gpio-regs.h

diff --git a/system/dev/gpio/mt-8167/mt8167-gpio-regs.h b/system/dev/gpio/mt-8167/mt8167-gpio-regs.h
index be1a049..bfc2d80 100644
--- a/system/dev/gpio/mt-8167/mt8167-gpio-regs.h
+++ b/system/dev/gpio/mt-8167/mt8167-gpio-regs.h
@@ -11,13 +11,14 @@
 template <>
 template <>
 __WEAK uint16_t ddk::MmioBuffer::Read<uint16_t>(zx_off_t offs) const {
-    return Read<uint16_t>(offs);
+    return *reinterpret_cast<volatile uint16_t*>(ptr_ + offs);
 }
 
 template <>
 template <>
 __WEAK void ddk::MmioBuffer::Write<uint16_t>(uint16_t val, zx_off_t offs) const {
-    Write<uint16_t>(val, offs);
+    *reinterpret_cast<volatile uint16_t*>(ptr_ + offs) = val;
+    hw_mb();
 }
 
 namespace {