If the twl4030_write_script function gets called with
a zero length argument, its return value does not
get set. We know that all scripts have a nonzero
length, but returning an error in case they ever
do is probably appropriate.

Without this patch, building omap2plus_defconfig results in:

drivers/mfd/twl4030-power.c: In function 'load_twl4030_script':
drivers/mfd/twl4030-power.c:414:5: error: 'err' may be used uninitialized in this function
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: "Kristo, Tero" <t-kristo@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>