MMC: meson: avoid possible NULL dereference

No actual segmentation faults were observed but the coding is
at least inconsistent.

irqreturn_t meson_mmc_irq():

We should not dereference host before checking it.

meson_mmc_irq_thread():

If cmd or mrq are NULL we should not dereference them after
writing a warning.

Fixes: 51c5d844 MMC: meson: initial support for GX platforms
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>