staging: xillybus: Register's address offset notation update

In iowrite32() calls, the address of registers was expressed as e.g.
&ep->registers[fpga_msg_ctrl_reg]. This changes to the more common format
e.g. ep->registers + fpga_msg_ctrl_reg.

There is no functional change.

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