bq27x00_battery: Fix flag register read

When reading flags, bq27x00_read() argument is inverted and causes
reads 2 of bytes for bq27200 and 1 byte for bq27500, while their register
sizes are 1 and 2 bytes respectively. This causes bq27500 upper flag
bits always to be returned as 0, causing full charge state to never be
reported correctly, so fix it.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>