wil6210: fix for "runtime PM when interface is down"

The original change used readl and writel which can
be replaced with readl_relaxed and writel_relaxed,
since we take care of memory barriers in the code.

Change-Id: I4e653f51c24ee4b2fa3d1aee38574fa097424e63
Signed-off-by: Lior David <liord@codeaurora.org>