Fpga: Documentation: Replace deprecated :c:func: Usage

Replace :c:func: with func() as the previous usage is deprecated.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Link: https://lore.kernel.org/r/20200812180224.24810-1-puranjay12@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>