drivers/video/exynos/exynos_mipi_dsi.c: fix an error check condition

Checking an unsigned variable for negative value returns false.  Hence use
the macro to fix it.

Fixes the following smatch warning:

  drivers/video/exynos/exynos_mipi_dsi.c:417 exynos_mipi_dsi_probe() warn: unsigned 'dsim->irq' is never less than zero.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Donghwa Lee <dh09.lee@samsung.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>