ath6kl: Use vmalloc instead of kmalloc for fw

Sometimes it has been observed that allocating a contiguous memory
of more than 100K fails with kmalloc. This has been modified to
use vmalloc instead.

Signed-off-by: PingYang Zhang <pingzhan@qca.qualcomm.com>
Signed-off-by: Vivek Natarajan <nataraja@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>