leds: don't disable blinking when writing the same value to delay_on or delay_off

Function led_set_software_blink() assumes that blink timer is still running,
but commit 488bc35bf40df89d37486c1826b178a2fba36ce7 introduced disabling
of blink timer before each call to led_set_software_blink().

Correct led_software_blink():
1) remove protection against reprogramming blink timer to the same values,
   because it only disables blinking now,
2) remove unnecessary call to led_stop_software_blink().

Signed-off-by: Rafal Prylowski <prylowski@metasoft.pl>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>