plugins/pppoe: Use value from pppoe-padi-* options

plugins/pppoe: Use value from pppoe-padi-* options

Reading the values of pppoe_padi_timeout and pppoe_padi_attempts in
PPPOEInitDevice() means that they get sampled when the parsing the
ethernet device name.  If the user provides the pppoe-padi-attempts or
pppoe-padi-timeout option after the ethernet device name, the value
given is effectively ignored.

Instead, read those variables in pppoe_check_options, which is called
after all options have been parsed, so that any user-specified values
don't get missed.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>