genericirq.rst: Remove :c:func:`...` in code blocks

In code blocks, :c:func:`...` annotations don't result in
cross-references. Instead, they are rendered verbatim.  Remove these
broken annotations, and mark function calls with parentheses() again.

Fixes: 76d40fae

 ("genericirq.rst: add cross-reference links and use monospaced fonts")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>