mfd: fix undefined twl4030-power resconfig value checks

The code tries to skip values initialized with -1, but since the values
are unsigned the comparison is always true.

The patch eliminates the following compiler warnings:

drivers/mfd/twl4030-power.c: In function 'twl4030_configure_resource':
drivers/mfd/twl4030-power.c:338: warning: comparison is always true due to
limited range of data type
drivers/mfd/twl4030-power.c:358: warning: comparison is always true due to
limited range of data type
drivers/mfd/twl4030-power.c:363: warning: comparison is always true due to
limited range of data type

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>