mailbox: Fix devm_ioremap_resource error detection code

devm_ioremap_resource returns an ERR_PTR value, not NULL,
on failure.

The Coccinelle semantic patch used to make this change is
as follows:
@@
expression e,e1;
statement S;
@@

*e = devm_ioremap_resource(...);
if (!e1) S

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>