memory: jz4780_nemc: Fix an error pointer vs NULL check in probe()

The devm_ioremap() function returns NULL on error, it doesn't return
error pointers.  This bug could lead to an Oops during probe.

Fixes: f046e4a3 ("memory: jz4780_nemc: Only request IO memory the driver will use")
Cc: <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20200803143607.GC346925@mwanda


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>