twl4030_charger: fix warnings

Fix warnings emitted by some versions of gcc:
drivers/power/twl4030_charger.c:490: warning: overflow in implicit constant conversion
drivers/power/twl4030_charger.c:498: warning: overflow in implicit constant conversion

While at it, also fix module_param permissions and a typo in my name.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>