staging: xillybus: Removed mmiowb() as iowrite32() is ordered

mmiowb() was used to make sure that iowrite32() take place in the correct
order, which is an unnecessary precuation.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>