mfd: ab8500-sysctrl: Let sysctrl driver work without pdata

A check for a valid plat->sysctrl was introduced in:

2377e52 mfd: ab8500-sysctrl: Error check clean up

but the driver works just fine even without that initialization data,
and enforcing it breaks existing platforms for no reason.

This patch removes the check and let the driver go ahead with probe.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>