bnxt_en: Check for zero dir entries in NVRAM.

If firmware goes into unstable state, HWRM_NVM_GET_DIR_INFO firmware
command may return zero dir entries. Return error in such case to
avoid zero length dma buffer request.

Fixes: c0c050c5 ("bnxt_en: New Broadcom ethernet driver.")
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>