bq27x00: Fix I2C dependency in KConfig

This patch fixes build failure(randconfig) of next-20130501. When config
I2C as m, BATTERY_BQ27x00 as y, here comes the failure. The driver depends
on I2C only if I2C is not disabled, as Lars commented. Last version of
this patch make the driver depend on I2C unconditionally.

Failure message:
drivers/built-in.o: In function `bq27x00_read_i2c':
bq27x00_battery.c:(.text+0x1082a7): undefined reference to `i2c_transfer'
drivers/built-in.o: In function `bq27x00_battery_init':
bq27x00_battery.c:(.init.text+0x6085): undefined reference to `i2c_register_driver'
bq27x00_battery.c:(.init.text+0x60c7): undefined reference to `i2c_del_driver'
drivers/built-in.o: In function `bq27x00_battery_exit':
bq27x00_battery.c:(.exit.text+0xbf0): undefined reference to `i2c_del_driver'
make: *** [vmlinux] Error 1

Signed-off-by: Xiong Zhou <jencce.kernel@gmail.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>