ata: libata-core: fix a doc warning

ata: libata-core: fix a doc warning

The docs toolchain doesn't recognise this pattern:

	@link->[hw_]sata_spd_limit

As it can't really process it. So, instead, let's mark it with
a literal block markup:

	``link->[hw_]sata_spd_limit``

in order to get rid of the following warning:

	./drivers/ata/libata-core.c:5974: WARNING: Unknown target name: "hw".

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9a21444df75c46095c4b1839d2061d19c9addcff.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>