cxgb4: halt chip before flashing PHY firmware image

When using firmware-assisted PHY firmware image write to flash,
halt the chip before beginning the flash write operation to allow
the running firmware to store the image persistently. Otherwise,
the running firmware will only store the PHY image in local on-chip
RAM, which will be lost after next reset.

Fixes: 4ee339e1 ("cxgb4: add support to flash PHY image")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>