package/linux-firmware: fix typo in variable name

Remove a trailing _ leftover from: c0d82bd3 (package/linux-firmware: install
license files with 'make legal-info')

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/linux-firmware/linux-firmware.mk

@@ -10,7 +10,7 @@ LINUX_FIRMWARE_SITE_METHOD = git
 
 # rt2501/rt61
 ifeq ($(BR2_PACKAGE_LINUX_FIRMWARE_RALINK_RT61),y)
-LINUX_FIRMWARE_FILES_ += rt2561.bin rt2561s.bin rt2661.bin
+LINUX_FIRMWARE_FILES += rt2561.bin rt2561s.bin rt2661.bin
 LINUX_FIRMWARE_ALL_LICENSE_FILES += LICENCE.ralink-firmware.txt
 endif