serial: max310x: fix NULL pointer dereference in I2C instantiation

In the Linux kernel, the following vulnerability has been resolved:

serial: max310x: fix NULL pointer dereference in I2C instantiation

When trying to instantiate a max14830 device from userspace:

echo max14830 0x60 > /sys/bus/i2c/devices/i2c-2/new_device

we get the following error:

Unable to handle kernel NULL pointer dereference at virtual address...

...

Call trace:

max310x_i2c_probe+0x48/0x170 [max310x]

i2c_device_probe+0x150/0x2a0

...

Add check for validity of devtype to prevent the error, and abort probe

with a meaningful error message.