efi_loader: fix CapsuleMax variable reporting

Currently the code that adds the CapsuleMax variable is under a
Kconfig named 'EFI_HAVE_CAPSULE_UPDATE.  Git history only shows a
single occurrence of that.  The IS_ENABLED should be checking for
EFI_HAVE_CAPSULE_SUPPORT

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>