linux-firmware: replace mkdir by install

linux-firmware: replace mkdir by install

if a setup is using RPM for packaging and there are multiple
recipes that install to ${nonarch_base_libdir}/firmware by using
install -d ${nonarch_base_libdir}/firmware, it will create installation
clashes on image install, as linux-firmware in before this patch
used mkdir -p, which creates different file mode bits (depending
on the current user's settings).

In a particular example
linux-fimware created /lib/firmware with 0600
while other-firmware-package created it with 0644
making the combination not installable by rpm backend

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>