Check for regmap_read() return code before using the read value in
following write in exynos5_switch_timing_regs().  Pass reading error
code to the callers.

This does not introduce proper error handling for such failed reads (and
obviously regmap_write() error is still ignored) because the driver
ignored this in all places.  Therefor it only fixes reported issue while
matching current driver coding style:

       drivers/memory/samsung/exynos5422-dmc.c: In function 'exynos5_switch_timing_regs':
    >> drivers/memory/samsung/exynos5422-dmc.c:216:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>