hostapd: keep previous patches when DRIVER_RTW set

Changed HOSTAPD_PATCH= to HOSTAPD_PATCH+= to keep previously added
patches.

Signed-off-by: Alexander Mukhin <alexander.i.mukhin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/hostapd/hostapd.mk

@@ -48,7 +48,7 @@ HOSTAPD_CONFIG_EDITS += 's/\#\(CONFIG_TLS=\).*/\1internal/'
 endif
 
 ifeq ($(BR2_PACKAGE_HOSTAPD_DRIVER_RTW),y)
-HOSTAPD_PATCH = https://github.com/pritambaral/hostapd-rtl871xdrv/raw/master/rtlxdrv.patch
+HOSTAPD_PATCH += https://github.com/pritambaral/hostapd-rtl871xdrv/raw/master/rtlxdrv.patch
 HOSTAPD_CONFIG_SET += CONFIG_DRIVER_RTW
 endif