power: qcom-charger: fix possible out of bounds access for GEN3 FG driver

Fix the following things in fg-util.c which is included by GEN3
FG driver:

- Possible out of bounds access in fg_sram_dfs_reg_write() when
  using bytes_read from sscanf
- Fix uninitialized usage of variable in write_next_line_to_log()

Change-Id: If9e7ba5632d1b5f99d91bda6276d9123c37e4dc7
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>