iwlwifi: use DEFINE_MUTEX() for mutex lock

mutex lock can be initialized automatically with DEFINE_MUTEX()
rather than explicitly calling mutex_init().

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210802172232.2a583d006739.Ia782c08525397b6430b1c877a870d688a7f3f494@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>