efi/apple-properties: Reinstate support for boolean properties

efi/apple-properties: Reinstate support for boolean properties

Since commit 4466bf82821b ("efi/apple-properties: use
PROPERTY_ENTRY_U8_ARRAY_LEN"), my MacBook Pro issues a -ENODATA error
when trying to assign EFI properties to the discrete GPU:

pci 0000:01:00.0: assigning 56 device properties
pci 0000:01:00.0: error -61 assigning properties

That's because some of the properties have no value.  They're booleans
whose presence can be checked by drivers, e.g. "use-backlight-blanking".

Commit 6e98503dba64 ("efi/apple-properties: Remove redundant attribute
initialization from unmarshal_key_value_pairs()") employed a trick to
store such booleans as u8 arrays (which is the data type used for all
other EFI properties on Macs):  It cleared the property_entry's
"is_array" flag, thereby denoting that the value is stored inline in the
property_entry.

Commit 4466bf82821b erroneously removed that trick.  It was probably a
little fragile to begin with.

Reinstate support for boolean properties by explicitly invoking the
PROPERTY_ENTRY_BOOL() initializer for properties with zero-length value.

Fixes: 4466bf82821b ("efi/apple-properties: use PROPERTY_ENTRY_U8_ARRAY_LEN")
Cc: <stable@vger.kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/be958bda75331a011d53c696d1deec8dccd06fd2.1609388549.git.lukas@wunner.de
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>