jz4740-battery: Check if platform_data is supplied

Currently platform_data is dereferenced without checking whether it
is actually set, which can lead to kernel crashes.
This patch adds a check which will abort the drivers probe function
gracefully if no platform_data is supplied.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>