staging: rtl8712: remove wrapper function _init_workitem()

The inline function _init_workitem() is just a wrapper around INIT_WORK().
Oddly, this wrapper accepts 3 parameters while INIT_WORK() only accepts 2.
The third parameter is unused.

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>