iwlwifi: Add comments about MAC_ACCESS_REQ

	/* this bit wakes up the NIC */

	_iwl_set_bit(priv, CSR_GP_CNTRL, CSR_GP_CNTRL_REG_FLAG_MAC_ACCESS_REQ);

	/*

	 * These bits say the device is running, and should keep running for

	 * at least a short while (at least as long as MAC_ACCESS_REQ stays 1),

	 * but they do not indicate that embedded SRAM is restored yet;

	 * 3945 and 4965 have volatile SRAM, and must save/restore contents

	 * to/from host DRAM when sleeping/waking for power-saving.

	 * Each direction takes approximately 1/4 millisecond; with this

	 * overhead, it's a good idea to grab and hold MAC_ACCESS_REQUEST if a

	 * series of register accesses are expected (e.g. reading Event Log),

	 * to keep device from sleeping.

	 *

	 * CSR_UCODE_DRV_GP1 register bit MAC_SLEEP == 0 indicates that

	 * SRAM is okay/restored.  We don't check that here because this call

	 * is just for hardware register access; but GP1 MAC_SLEEP check is a

	 * good idea before accessing 3945/4965 SRAM (e.g. reading Event Log).

	 *

	 * 5000 series and later (including 1000 series) have non-volatile SRAM,

	 * and do not save/restore SRAM when power cycling.

	 */

	ret = _iwl_poll_bit(priv, CSR_GP_CNTRL,

			   CSR_GP_CNTRL_REG_VAL_MAC_ACCESS_EN,

			   (CSR_GP_CNTRL_REG_FLAG_MAC_CLOCK_READY |