docs: nvmem: remove function parameters (fixes hyperlink generation)

Adding a parameter is not particularly useful here, and it is definitely
not done elsewhere. It also prevents the generation of a hyperlink to the
kernel-doc documentation. Removing the parameter to enable hyperlinks and
make the style coherent.

Signed-off-by: Luca Ceresoli <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/nvmem.rst

···
 nvmem configuration to nvmem_register(), on success core would return a valid
 nvmem_device pointer.
 
-nvmem_unregister(nvmem) is used to unregister a previously registered provider.
+nvmem_unregister() is used to unregister a previously registered provider.
 
 For example, a simple nvram case::