serial: max3100: Update uart_driver_registered on driver removal

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

serial: max3100: Update uart_driver_registered on driver removal

The removal of the last MAX3100 device triggers the removal of

the driver. However, code doesn't update the respective global

variable and after insmod — rmmod — insmod cycle the kernel

oopses:

max3100 spi-PRP0001:01: max3100_probe: adding port 0

BUG: kernel NULL pointer dereference, address: 0000000000000408

...

RIP: 0010:serial_core_register_port+0xa0/0x840

...

max3100_probe+0x1b6/0x280 [max3100]

spi_probe+0x8d/0xb0

Update the actual state so next time UART driver will be registered

again.

Hugo also noticed, that the error path in the probe also affected

by having the variable set, and not cleared. Instead of clearing it

move the assignment after the successfull uart_register_driver() call.