leds: leds-qpnp: correct brightness value for MPP based LEDs

MPP ports provide LEDs with current from 5mA to 40mA with a resolution
of 5mA. Due to this PMIC feature, MPP based LED intensity should be
rounded to reflect actual hardware status.

Change-Id: I306f19df0dd455dbaad9a364fb0df5694015d9ea
Signed-off-by: Chun Zhang <chunz@codeaurora.org>