Docs: usb: remove :c:func: for usb_register and usb_deregister

remove :c:func: for usb_register and usb_deregister
Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/0ace789dfbe2d4562c27d374afa5ff078efe2261.1635138058.git.philipp.g.hortmann@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/writing_usb_driver.rst

@@ -84,7 +84,7 @@ this user-space interaction. The skeleton driver needs this kind of
 interface, so it provides a minor starting number and a pointer to its
 :c:type:`file_operations` functions.
 
-The USB driver is then registered with a call to :c:func:`usb_register`,
+The USB driver is then registered with a call to usb_register(),
 usually in the driver's init function, as shown here::
 
     static int __init usb_skel_init(void)
@@ -105,7 +105,7 @@ usually in the driver's init function, as shown here::
 
 
 When the driver is unloaded from the system, it needs to deregister
-itself with the USB subsystem. This is done with the :c:func:`usb_deregister`
+itself with the USB subsystem. This is done with usb_deregister()
 function::
 
     static void __exit usb_skel_exit(void)