serial: sc16is7xx: rename EFR mutex with generic name

serial: sc16is7xx: rename EFR mutex with generic name

This mutex is used as a lock when accessing registers that share the same
address space, not necessarily EFR registers.

For example, address 0x06 is shared by MSR, TCR and XOFF1 registers,
independently of EFR.

Rename the mutex with a more generic name to avoid misinterpreting its
usage.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://patch.msgid.link/20251027142957.1032073-3-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>