enic: fix build with gcc 4.7.2

enic: fix build with gcc 4.7.2

Compile warnings/errors was found on gcc 4.7.2 as follows. Variables
was reported of being used but uninitialized. Assigning an initial
value to it is needed.

lib/librte_pmd_enic/vnic/vnic_dev.c: In function vnic_dev_get_mac_addr:
lib/librte_pmd_enic/vnic/vnic_dev.c:393:16: error: a1 may be used uninitialized
	in this function [-Werror=uninitialized]
lib/librte_pmd_enic/vnic/vnic_dev.c:629:10: note: a1 was declared here
lib/librte_pmd_enic/vnic/vnic_dev.c: In function vnic_dev_set_mac_addr:
lib/librte_pmd_enic/vnic/vnic_dev.c:393:16: error: a1 may be used uninitialized
	in this function [-Werror=uninitialized]
lib/librte_pmd_enic/vnic/vnic_dev.c:980:10: note: a1 was declared here

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>