power: supply: bq27xxx: fix wrong errno when bus ops are unsupported

power: supply: bq27xxx: fix wrong errno when bus ops are unsupported

bq27xxx_write(), bq27xxx_read_block(), and bq27xxx_write_block()
return -EPERM when the bus callback pointer is NULL. A NULL callback
indicates the operation is not supported by the bus/driver,
not that permission is denied.

Return -EOPNOTSUPP instead of -EPERM when di->bus.write/
read_bulk/write_bulk is NULL.

Fixes: 14073f6614f6 ("power: supply: bq27xxx: Add bulk transfer bus methods")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Matt Ranostay <matt@ranostay.sg>
Link: https://patch.msgid.link/20251204083436.1367-1-vulab@iscas.ac.cn
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>