serial: max310x: improve interrupt handling

serial: max310x: improve interrupt handling

When there is a heavy load of receiving characters to all
four UART's, the warning 'Hardware RX FIFO overrun' is
sometimes detected.
The current implementation always service first the highest UART
until no more interrupt and then service another UART
(ex: UART3 will be serviced for as long as there are interrupts
for it, then UART2, etc).

This commit handle all individual interrupt sources before
reading the global IRQ register again.

This commit has also a nice side-effect of improving the efficiency
of the driver by reducing the number of reads of the global
IRQ register.

Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20250908-master-max310x-improve-interrupt-handling-v3-1-91985e82ba39@vaisala.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>