power: supply: max17040: fix null-ptr-deref in max17040_probe()

Add check the return value of devm_regmap_init_i2c(), otherwise
later access may cause null-ptr-deref as follows:

KASAN: null-ptr-deref in range [0x0000000000000360-0x0000000000000367]
RIP: 0010:regmap_read+0x33/0x170
Call Trace:
  max17040_probe+0x61b/0xff0 [max17040_battery]
 ? write_comp_data+0x2a/0x90
 ? max17040_set_property+0x1d0/0x1d0 [max17040_battery]
 ? tracer_hardirqs_on+0x33/0x520
 ? __sanitizer_cov_trace_pc+0x1d/0x50
 ? _raw_spin_unlock_irqrestore+0x4b/0x60
 ? trace_hardirqs_on+0x63/0x2d0
 ? write_comp_data+0x2a/0x90
 ? __sanitizer_cov_trace_pc+0x1d/0x50
 ? max17040_set_property+0x1d0/0x1d0 [max17040_battery]
 i2c_device_probe+0xa31/0xbe0

Fixes: 6455a8a8 ("power: supply: max17040: Use regmap i2c")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>