mfd: wcd9xxx-core: fix the device registeration check.

The current logic checks for only one byte id for device
registration. this is not sufficient to identify all
variants. fix this to check all 4 byte id registers.

Change-Id: I697a7b8c450b63efdde9b3b0f6716b191497ee63
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>