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>