maillbox: bcm-flexrm-mailbox: handle cmpl_pool dma allocation failure

Handle 'cmpl_pool' dma memory allocation failure.

Fixes: a9a9da47 ("mailbox: no need to check return value of debugfs_create functions")
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>