bnxt_en: Refactor coredump functions

The coredump functionality will be used by devlink health. Refactor
these functions that get coredump and coredump length. There is no
functional change, but the following checkpatch warnings were
addressed:

  - strscpy is preferred over strlcpy.
  - sscanf results should be checked, with an additional warning.

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>