sfc: Remove write permission from phy_type attribute

Driver probe currently results in:

WARNING: at drivers/base/core.c:576 device_create_file+0x57/0x7e()
Attribute phy_type: write permission without 'store'

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>