leds: leds-qpnp: Set proper flash duration

Flash duration is between 0ms and 1280ms in increments of 10ms and can
be stored in an 8-bit value. The current conversion formula to u8
results in shrinking of this value, which leads to a loss of
most-significant bit in flash duration. This erroneous calcuation
is corrected in the driver.

Change-Id: I43b486743703ed8007e2366e94ce145fdd4dc56e
CRs-Fixed: 582957
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>