serial: max310x: use separate regmap name for each port

serial: max310x: use separate regmap name for each port

Use a separate regmap name for each port so they can each have their own
debugfs entry, allowing to access each port registers independently.

For example, a four channels/ports device like the MAX14830 will have four
entries in its regmap debugfs:

$ find /sys/kernel/debug/regmap -type d | grep spi0.0
/sys/kernel/debug/regmap/spi0.0-port0
/sys/kernel/debug/regmap/spi0.0-port1
/sys/kernel/debug/regmap/spi0.0-port2
/sys/kernel/debug/regmap/spi0.0-port3

Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Link: https://lore.kernel.org/all/77f101f1-897d-4e6d-a8fd-27b818caf768@cesnet.cz/
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20240118152213.2644269-9-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>