leds: expresswire: Fix chip state breakage

leds: expresswire: Fix chip state breakage

It is possible to put the KTD2801 chip in an unknown/undefined state by
changing the brightness very rapidly (for example, with a brightness
slider). When this happens, the brightness is stuck on max and cannot be
changed until the chip is power cycled.

Fix this by disabling interrupts while talking to the chip. While at it,
make expresswire_power_off() use fsleep() and also unexport some
functions meant to be internal.

Fixes: 1368d06dd2c9 ("leds: Introduce ExpressWire library")
Tested-by: Karel Balej <balejk@matfyz.cz>
Signed-off-by: Duje Mihanović <duje@dujemihanovic.xyz>
Link: https://patch.msgid.link/20251217-expresswire-fix-v2-1-4a02b10acd96@dujemihanovic.xyz
Signed-off-by: Lee Jones <lee@kernel.org>