rtl8821cu: disable CONFIG_MP_VHT_HW_TX_MODE for ARM64

rtl8812au driver has some portions of code with floating point (only used when CONFIG_MP_VHT_HW_TX_MODE is enabled).
But on arm64 there are no support for soft floating-point and HW fp is disabled in kernel.
So the only way to disable CONFIG_MP_VHT_HW_TX_MODE functionality.

This fix project-openwrt/openwrt#53.

package/ctcgfw/rtl8821cu/Makefile

@@ -41,6 +41,11 @@ EXTRA_KCONFIG:=\
 	CONFIG_RTL8821CU=m \
 	USER_MODULE_NAME=rtl8821cu
 
+ifeq ($(ARCH),aarch64)
+       EXTRA_KCONFIG+= CONFIG_MP_VHT_HW_TX_MODE=n
+endif
+
+
 EXTRA_CFLAGS:= \
 	-DRTW_USE_CFG80211_STA_EVENT \
 	-DCONFIG_RADIO_WORK \