iwlwifi: pcie: use jiffies for memory read spin time limit

There's no reason to use ktime_get() since we don't need any better
precision than jiffies, and since we no longer disable interrupts
around this code (when grabbing NIC access), jiffies will work fine.
Use jiffies instead of ktime_get().

This cleanup is preparation for the following patch "iwlwifi: pcie: reschedule
in long-running memory reads". The code gets simpler with the weird clock use
etc. removed before we add cond_resched().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130253.621c948b1fad.I3ee9f4bc4e74a0c9125d42fb7c35cd80df4698a1@changeid