wlan: use vos_mem_malloc to allocate memory

kzalloc() is used to allocate memory for pkt_stats_buffers and
vos_mem_free() is used to free memory. This results in memory leak.

To address this use vos_mem_malloc() to allocate memory.

Change-Id: I79da24da7002829b2d1721ec479f907d54a01a8a
CRs-Fixed: 2733628