Staging: fwserial: remove unused function fill_unplug_req

The function fill_unplug_req is not used anywhere in the kernel, so
remove it. Done using Coccinelle.

@r1@
identifier func;
type T;
@@
static T func(...)
{
...
}

@r@
identifier r1.func;
@@
func

@delete depends on !r@
identifier r1.func;
type r1.T;
@@
- static T func(...){...}

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>