bnxt_en: Fix return code to "flash_device".

When NVRAM directory is not found, return the error code
properly as per firmware command failure instead of the hardcode
-ENOBUFS.

Fixes: 3a707bed ("bnxt_en: Return -EAGAIN if fw command returns BUSY")
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>