sfc: mtd: Use MTD_FAIL_ADDR_UNKNOWN instead of 0xffffffff

As of bb0eb217, MTD_FAIL_ADDR_UNKNOWN should be used to indicate mtd
erase failure not specific to any particular block.

Use MTD_FAIL_ADDR_UNKNOWN instead of 0xffffffff when setting
'erase->fail_addr' in 'efx_mtd_erase()'.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>