serial: 8250_exar: Clear interrupts before registering handler

serial: 8250_exar: Clear interrupts before registering handler

While now there is no issue if IRQ is fired before we clearing
the interrupts as the handler does the same, but strictly speaking
it might be problematic if IRQ handler wants to do something more.

Move clearing interrupt code to be called before registering the
IRQ handler.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240219150627.2101198-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>