power: supply: sysfs: Remove duplicate NUL termination

power: supply: sysfs: Remove duplicate NUL termination

GCC 15's new -Wunterminated-string-initialization notices that one of
the sysfs attr strings would lack the implicit trailing NUL byte during
initialization:

drivers/power/supply/power_supply_sysfs.c:183:57: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (32 chars into 31 available) [-Wunterminated-string-initialization]
  183 |         POWER_SUPPLY_ATTR(CHARGE_CONTROL_START_THRESHOLD),
      |                                                         ^
drivers/power/supply/power_supply_sysfs.c:36:23: note: in definition of macro '_POWER_SUPPLY_ATTR'
   36 |         .attr_name = #_name "\0",               \
      |                       ^~~~~
drivers/power/supply/power_supply_sysfs.c:183:9: note: in expansion of macro 'POWER_SUPPLY_ATTR'
  183 |         POWER_SUPPLY_ATTR(CHARGE_CONTROL_START_THRESHOLD),
      |         ^~~~~~~~~~~~~~~~~

However, the macro used was explicitly adding a trailing NUL byte (which
is not needed). Remove this to avoid the GCC warning. No binary
differences are seen after this change (there was always run for a NUL
byte, it's just that the _second_ NUL byte was getting truncated).

Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250416222740.work.569-kees@kernel.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>