wifi: rtw89: Add missing check for alloc_workqueue

Add check for the return value of alloc_workqueue since it may return
NULL pointer.
Moreover, add destroy_workqueue when rtw89_load_firmware fails.

Fixes: e3ec7017 ("rtw89: add Realtek 802.11ax driver")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230104142901.1611-1-jiasheng@iscas.ac.cn