mailbox: qcom-apcs: fix max_register value

The mailbox length is 0x1000 hence the max_register value is 0xFFC.

Fixes: c6a8b171

 ("mailbox: qcom: Convert APCS IPC driver to use
regmap")
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>