leds: qpnp-flash-v2: Modify current code calculation

Currently, the code calculated for a desired target current and
IRES is rounded up since DIV_ROUND_UP is used in the calculation.
With a higher IRES (12.5 mA), code can be configured to a higher
value. Fix this by using DIV_ROUND_CLOSEST so that the optimal
code can be obtained.

Change-Id: I51c1b15fff3ff2a23cb256f2ae1d341f5271adf2
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>