efi: efibc: check for efivars write capability

Branden reports that commit

  f88814cc ("efi/efivars: Expose RT service availability via efivars abstraction")

regresses UEFI platforms that implement GetVariable but not SetVariable
when booting kernels that have EFIBC (bootloader control) enabled.

The reason is that EFIBC is a user of the efivars abstraction, which was
updated to permit users that rely only on the read capability, but not on
the write capability. EFIBC is in the latter category, so it has to check
explicitly whether efivars supports writes.

Fixes: f88814cc

 ("efi/efivars: Expose RT service availability via efivars abstraction")
Tested-by: Branden Sherrell <sherrellbc@gmail.com>
Link: https://lore.kernel.org/linux-efi/AE217103-C96F-4AFC-8417-83EC11962004@gmail.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>