mwlwifi: use PKG_SOURCE_DATE instead of version

mwlwifi: use PKG_SOURCE_DATE instead of version

Using PKG_SOURCE_DATE instead of PKG_VERSION will make the build system
generate the version based on the date and the git hash. This way the
tar file name changes when the git hash changes and this avoids problems
when someone forgets to change the version, but changes the git hash.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>