bnxt_en: Refactor bnxt_flash_nvram.

Refactor bnxt_flash_nvram() into __bnxt_flash_nvram() that takes an
additional dir_item_len parameter.  The new function will be used
in subsequent patches with the dir_item_len parameter set to create
the UPDATE directory during flashing.

Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>