docs: driver-api: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/095b04bff6d49b4097382398bb91102eaa3f0fd3.1623824363.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/ioctl.rst

@@ -34,7 +34,7 @@ _IO/_IOR/_IOW/_IOWR
 
 type
    An 8-bit number, often a character literal, specific to a subsystem
-   or driver, and listed in :doc:`../userspace-api/ioctl/ioctl-number`
+   or driver, and listed in Documentation/userspace-api/ioctl/ioctl-number.rst
 
 nr
   An 8-bit number identifying the specific command, unique for a give