cxgb4: Fix t4_flash_erase_sectors() to throw an error when requested to erase sectors which aren't in the FLASH

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>