cxgb4: Fix FW flash logic using ethtool

Use t4_fw_upgrade instead of t4_load_fw to write firmware into FLASH, since
t4_load_fw doesn't co-ordinate with the firmware and the adapter can get hosed
enough to require a power cycle of the system.

Based on original work by Casey Leedom <leedom@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>