leds: Avoid division by zero in kernel ;)

In kernel log, it says 'Division by zero in kernel'.
And showing debug messages.

<4>[  546.621852] [<c0014340>] (unwind_backtrace+0x0/0xdc) from [<c02a7560>] (Ldiv0+0x8/0x10)
<4>[  546.622035] [<c02a7560>] (Ldiv0+0x8/0x10) from [<c050cae8>] (pm8xxx_adjust_brightness+0x38/0x54)
<4>[  546.622310] [<c050cae8>] (pm8xxx_adjust_brightness+0x38/0x54) from [<c050d048>] (__pm8xxx_update_led+0x28/0x494)
<4>[  546.622615] [<c050d048>] (__pm8xxx_update_led+0x28/0x494) from [<c050bdc4>] (led_brightness_store+0xc8/0xe0)
<4>[  546.622890] [<c050bdc4>] (led_brightness_store+0xc8/0xe0) from [<c017c768>] (sysfs_write_file+0x108/0x13c)
<4>[  546.623195] [<c017c768>] (sysfs_write_file+0x108/0x13c) from [<c012b2cc>] (vfs_write+0xac/0x160)
<4>[  546.623348] [<c012b2cc>] (vfs_write+0xac/0x160) from [<c012b618>] (sys_write+0x38/0x64)
<4>[  546.623653] [<c012b618>] (sys_write+0x38/0x64) from [<c000dc3c>] (__sys_trace_return+0x0/0x24)

This patch fix this issue.