serial: sc16is7xx: refactor EFR lock

Move common code for EFR lock/unlock of mutex into functions for code reuse
and clarity.

With the addition of old_lcr, move irda_mode within struct sc16is7xx_one to
reduce memory usage:
    Before: /* size: 752, cachelines: 12, members: 10 */
    After:  /* size: 744, cachelines: 12, members: 10 */

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231221231823.2327894-17-hugo@hugovil.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>