mailbox: th1520: Fix a NULL vs IS_ERR() bug

The devm_ioremap() function doesn't return error pointers, it returns
NULL.  Update the error checking to match.

Fixes: 5d4d263e ("mailbox: Introduce support for T-head TH1520 Mailbox driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>