Forced Fast Charge v1.4a

- return "invalid argument" when writing a wrong value in sysfs tunables

SysFS interface :

 * /sys/kernel/fast_charge/force_fast_charge (rw)
 *
 *   0 - disabled (default)
 *   1 - substitute AC to USB
 *   2 - use custom mA configured through sysfs interface (see below)
 *
 * /sys/kernel/fast_charge/ac_charge_level (rw)
 *
 *   rate at which to charge when on AC (1.0A to 1.5A)
 *
 * /sys/kernel/fast_charge/usb_charge_level (r/w)
 *
 *   rate at which to charge when on USB (0.475A to 1.0A)
 *
 * /sys/kernel/fast_charge/wireless_charge_level (r/w)
 *
 *   rate at which to charge when on wireless (0.475A to 1.0A)
 *
 * /sys/kernel/fast_charge/failsafe (rw)
 *
 *   0 - disabled - allow anything up to 2.1A to be used as AC / USB custom current
 *   1 - enabled  - behaviour as described above (default)
 *
 * /sys/kernel/fast_charge/version (ro)
 *
 *   display fast charge version information