mfd: iqs62x: Remove unused bit mask

The register write that performed a mandatory soft reset during
probe was removed during development of the driver, however the
IQS62X_SYS_SETTINGS_SOFT_RESET bit mask was left behind. Remove
it to keep stray macros out of the driver.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>