wcn36xx: Move hal_buf allocation to devm_kmalloc in probe

Right now wcn->hal_buf is allocated in wcn36xx_start(). This is a problem
since we should have setup all of the buffers we required by the time
ieee80211_register_hw() is called.

struct ieee80211_ops callbacks may run mac_start() and therefore
wcn->hal_buf must be initialized prior to mac_start().

This is easily remediated by moving the allocation to probe() taking the
opportunity to tidy up freeing memory by using devm_kmalloc().

Fixes: 8e84c258 ('wcn36xx: mac80211 driver for Qualcomm WCN3660/WCN3680
hardware')

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>