staging/fwserial: Annotate rcu pointers with __rcu

Fixes these sparse warnings:
drivers/staging/fwserial/fwserial.c:430:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:699:30: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:802:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:898:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:1842:14: sparse: incompatible types in comparison expression (different address spaces)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>