serial: pl011: protect attribute read from NULL platform data struct

serial: pl011: protect attribute read from NULL platform data struct

It's completely feasible that platform data will be empty i.e. when
booting with Device Tree with no device AUXDATA. So we must protect
it's use in these use-cases, or risk a kernel Oops.

Cc: Russell King <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Signed-off-by: Lee Jones <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>