regulator: core: Use fsleep() to get best sleep mechanism

_regulator_delay_helper() implements the recommondation of the outdated
documentation which sleep mechanism should be used. There is already a
function in place which does everything and also maps to reality called
fsleep().

Use fsleep() directly.

Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20241014-devel-anna-maria-b4-timers-flseep-v3-11-dc8b907cb62f@linutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>