regulator: ad5398: change enable bit name to improve readibility

regulator: ad5398: change enable bit name to improve readibility

The mask name AD5398_CURRENT_EN_MASK is misleading, as it implies that
setting bit 16 of the AD5398 enables current flow. In fact, setting this
bit prevents current flow, due to this bit being a software power down
control. This bit is referred to as "soft power down" in the datasheet.
As such, change the name of the bit and modify its use in the driver to
make the regulator more intuitively usable.

(When calling ad5398_enable, current will start flowing, and vice
versa).

Signed-off-by: Isaac Scott <isaac.scott@ideasonboard.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Link: https://patch.msgid.link/20250128173143.959600-2-isaac.scott@ideasonboard.com
Signed-off-by: Mark Brown <broonie@kernel.org>