mailbox: bcm-flexrm-mailbox: using dev_get_drvdata directly

Several drivers cast a struct device pointer to a struct
platform_device pointer only to then call platform_get_drvdata().
To improve readability, these constructs can be simplified
by using dev_get_drvdata() directly.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>