serial: sc16is7xx: define common register access function

serial: sc16is7xx: define common register access function

Rename lock/unlock functions to make it more generic and applicable to both
the Enhanced register set and the Special register set.

Use this new generic function when accessing the Special register set in
sc16is7xx_set_baud(), and when accessing the Enhanced register set in
sc16is7xx_set_termios() and sc16is7xx_probe().

This helps readability and also avoid to make future mistakes when
accessing these obfuscated registers.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://patch.msgid.link/20251027142957.1032073-4-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>