'struct regulator_desc' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

This also makes mpq7932_regulators_desc consistent with
mpq7932_regulators_desc_one which is already a "static const struct
regulator_desc".

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   3516	   2264	      0	   5780	   1694	drivers/hwmon/pmbus/mpq7932.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   5396	    384	      0	   5780	   1694	drivers/hwmon/pmbus/mpq7932.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Message-ID: <c0585a07547ec58d99a5bff5e02b398114bbe312.1725784343.git.christophe.jaillet@wanadoo.fr>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>