brcmsmac: mac80211_if: Fix a resource leak in an error handling path

If 'brcms_attach()' fails, we must undo the previous 'ieee80211_alloc_hw()'
as already done in the remove function.

Fixes: 5b435de0 ("net: wireless: add brcm80211 drivers")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/8fbc171a1a493b38db5a6f0873c6021fca026a6c.1620852921.git.christophe.jaillet@wanadoo.fr