dpll: fix build failure due to rcu_dereference_check() on unknown type

dpll: fix build failure due to rcu_dereference_check() on unknown type

Tasmiya reports that their compiler complains that we deref
a pointer to unknown type with rcu_dereference_rtnl():

include/linux/rcupdate.h:439:9: error: dereferencing pointer to incomplete type ‘struct dpll_pin’

Unclear what compiler it is, at the moment, and we can't report
but since DPLL can't be a module - move the code from the header
into the source file.

Fixes: 0d60d8df6f49 ("dpll: rely on rcu for netdev_dpll_pin()")
Reported-by: Tasmiya Nalatwad <tasmiya@linux.vnet.ibm.com>
Link: https://lore.kernel.org/all/3fcf3a2c-1c1b-42c1-bacb-78fdcd700389@linux.vnet.ibm.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240229190515.2740221-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>