mmc: meson-gx: remove useless lock

[ Upstream commit 83076d2268c72d123f3d1eaf186a9f56ec1b943a ]

The spinlock is only used within the irq handler so it does not
seem very useful.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Stable-dep-of: 3c40eb814532 ("mmc: meson-gx: remove redundant mmc_request_done() call from irq context")
Signed-off-by: Sasha Levin <sashal@kernel.org>