Mailbox: Fix return value check in pcc_init()

In case of error, the function platform_create_bundle() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>

diff --cc drivers/mailbox/pcc.c
Simple merge