bq27x00_battery: use unaligned access helper

Remove hand-rolled get_unaligned_be16, this points to a possible bug as
bq27x00_read does another endian byteswap which sparse notices:

drivers/power/bq27x00_battery.c:81:14: warning: cast to restricted __be16
Which should probably be checked.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>