mailbox: pcc: Avoid using the uninitialized variable 'dev'

Smatch static checker warns:

  |  drivers/mailbox/pcc.c:292 pcc_mbox_request_channel()
  |  error: uninitialized symbol 'dev'.

Fix the same by using pr_err instead of dev_err as the variable 'dev'
is uninitialized at that stage.

Fixes: ce028702 ("mailbox: pcc: Move bulk of PCCT parsing into pcc_mbox_probe")
Cc: Jassi Brar <jassisinghbrar@gmail.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>