kobject: increase allowed number of uevent variables

SBS battery driver exposes 32 power supply properties now,
which will result in uevent failure on (un)plugging the
battery. Other drivers (e.g. bq27xxx) are also coming close
to this limit, so increase it.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>