configure: check for $download value properly

If configure was invoked with --disable-download and git
submodules were not checked out a warning is produced and the
configure script fails. But the $download variable (which
reflects the enable/disable download argument) is checked for in
a weird fashion:

  test -f "$download" = disabled

Drop the '-f' to check for the actual value of the variable.

Fixes: 2019cabf ("meson: subprojects: replace submodules with wrap files", 2023-06-06)
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>