spi: <linux/spi/spi.h>: add missing struct kernel-doc entry

spi: <linux/spi/spi.h>: add missing struct kernel-doc entry

Fix kernel-doc warning in spi.h by adding the missing kernel-doc entry
and also correct the original comment so that they both indicate the
correct polarity of the flag.

../include/linux/spi/spi.h:673: warning: Function parameter or member 'devm_allocated' not described in 'spi_controller'

Fixes: 354210552735 ("spi: Fix use-after-free with devm_spi_alloc_*")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: William A. Kennington III <wak@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Cc: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/20210628210520.5712-1-rdunlap@infradead.org
Signed-off-by: Mark Brown <broonie@kernel.org>