i40e: fix build with some toolchains

i40e: fix build with some toolchains

Compile warning which is treated as error occurs on Oracle Linux
(kernel 2.6.39, gcc 4.4.7) as below, or RHEL, CentOS. Aliasing
'struct i40e_aqc_debug_reg_read_write' should be avoided. Use the
elements inside that structure directly can fix the issue.

lib/librte_pmd_i40e/i40e_ethdev.c: In function 'eth_i40e_dev_init':
lib/librte_pmd_i40e/i40e_ethdev.c:5318: error: dereferencing pointer
  'cmd' does break strict-aliasing rules
lib/librte_pmd_i40e/i40e_ethdev.c:5314: note: initialized from here

Signed-off-by: Helin Zhang <helin.zhang@intel.com>