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

drivers/ata/libata-core.c

@@ -5209,7 +5209,7 @@ void ata_link_init(struct ata_port *ap, struct ata_link *link, int pmp)
  *	sata_link_init_spd - Initialize link->sata_spd_limit
  *	@link: Link to configure sata_spd_limit for
  *
- *	Initialize @link->[hw_]sata_spd_limit to the currently
+ *	Initialize ``link->[hw_]sata_spd_limit`` to the currently
  *	configured value.
  *
  *	LOCKING: