bnxt_en: Refactor coredump functions

bnxt_en: Refactor coredump functions

[ Upstream commit 9a575c8c25ae2372112db6d6b3e553cd90e9f02b ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>