mfd: da9053: Fix compiler warning message for uninitialised variable

Fix compiler warning caused by an uninitialised variable inside
da9052_group_write() function. Defaulting the value to zero covers
the trivial case.

Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>