power: bq27421: refactor the fuel gauge driver

Simplify and clean the duplicated functions, and
replace a polling in fuel gauge reading to power
supply framework triggered reading.

Change-Id: I94d4afd5783402f26bc5af8f0b256072085d86c9
Signed-off-by: choongryeol.lee <choongryeol.lee@lge.com>
Signed-off-by: Devin Kim <dojip.kim@lge.com>