arm-trusted-firmware-mediatek: fix copy&paste error in Makefile

When adding builds for MT7981 the related Makefile sections for MT7986
have apparently been copied, but in one instance the rename from 7986 to
7981 has been omitted. Fix that now.

Fixes: 602cb4f325 ("arm-trusted-firmware-mediatek: add build for MT7981 DDR3")
Signed-off-by: Daniel Golle <[email protected]>

package/boot/arm-trusted-firmware-mediatek/Makefile

@@ -118,7 +118,7 @@ define Trusted-Firmware-A/mt7981-sdmmc-ddr3
   DDR_TYPE:=ddr3
 endef
 
-define Trusted-Firmware-A/mt7986-snand-ddr3
+define Trusted-Firmware-A/mt7981-snand-ddr3
   NAME:=MediaTek MT7981 (SPI-NAND via SNFI, DDR3)
   BOOT_DEVICE:=snand
   BUILD_SUBTARGET:=filogic
@@ -347,7 +347,7 @@ TFA_TARGETS:= \
 	mt7981-emmc-ddr3 \
 	mt7981-nor-ddr3 \
 	mt7981-sdmmc-ddr3 \
-	mt7986-snand-ddr3 \
+	mt7981-snand-ddr3 \
 	mt7981-spim-nand-ddr3 \
 	mt7986-emmc-ddr3 \
 	mt7986-nor-ddr3 \