mfd: wm831x: Remove redundant !pdata checks

Since a copy of the pdata was added into the core struct in
commit f6dd8449 ("mfd: wm831x: Add basic device tree binding")
the pdata pointer in probe can no longer be NULL. As such remove
the redundant checks for this case.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>