input: synaptics_fw_update: fix insufficient bounds checking

Possible values of config_area are between 0 and 3. The patch
adds bounds checking in fwu_sysfs_config_area_store() function.
Also use kstrtou16() for parsing the config_area.

Change-Id: Ia0b58f1b5a359c67f420012876431dac920ecfbd
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>