bnxt_en: move coredump functions into dedicated file

Change bnxt_get_coredump() and bnxt_get_coredump_length() to non-static
functions.

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>