netpoll: Extract carrier wait function

Extract the carrier waiting logic into a dedicated helper function
netpoll_wait_carrier() to improve code readability and reduce
duplication in netpoll_setup().

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250618-netpoll_ip_ref-v1-1-c2ac00fe558f@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>