linux/linux.mk: fixed downloading kernel patches

Patches located at ftp or http(s) URLs were downloaded using DOWNLOAD
macro. For example, if linux source was located at external git
repository, DOWNLOAD macro uses git scheme as well and buildroot
tried to downlod a path using DOWNLOAD_GIT macro. As a result, nothing
was downloaded and build siletly passes.

Patches located at mentioned URLs is now downloaded directly with
DOWNLOAD_WGET macro.

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