ixgbe: fix reading of the buffer returned by the firmware

This patch fixes some issues found in the buffer read portion of
ixgbe_host_interface_command()

- use `bi` as the buffer index counter instead of `i`
- add conversion to native cpu byte ordering on register read
- fix conversion from bytes to dword
- use dword_len instead of buf_len when reading the register

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>