tty: serial: qcom_geni_serial: Remove use of *_relaxed() and mb()

[ Upstream commit 9e06d55f7b856bfaf82036b50072600b21e52d20 ]

A frequent side comment has been to remove the use of writel_relaxed,
readl_relaxed, and mb. This reduces driver complexity and the _relaxed
variants were not known to provide any noticeable performance benefit.

Signed-off-by: Ryan Case <ryandcase@chromium.org>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>