bnxt_en: take coredump_record structure off stack

The bnxt_coredump_record structure is very long, causing a warning
about possible stack overflow on 32-bit architectures:

drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c: In function 'bnxt_get_coredump':
drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c:2989:1: error: the frame size of 1188 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

I could not see any reason to operate on an on-stack copy of the
structure before copying it back into the caller-provided buffer, which
also simplifies the code here.

Fixes: 6c5657d0 ("bnxt_en: Add support for ethtool get dump.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>