regmap: add WARN_ONCE when invalid mask is provided to regmap_field_init()

In regmap_field_init() when a invalid mask is provided it still
initializes with any warnings.

An example of this is when the LSB is greater than MSB a mask of zero
is produced.

WARN_ONCE() is not ideal for this but requires less changes to core regmap
code.

Cc: Mark Brown <broonie@kernel.org>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Matt Ranostay <mranostay@ti.com>
Link: https://lore.kernel.org/r/20220708013125.313892-1-mranostay@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -1298,6 +1298,9 @@ static void regmap_field_init(struct regmap_field *rm_field,
 	rm_field->reg = reg_field.reg;
 	rm_field->shift = reg_field.lsb;
 	rm_field->mask = GENMASK(reg_field.msb, reg_field.lsb);
+
+	WARN_ONCE(rm_field->mask == 0, "invalid empty mask defined\n");
+
 	rm_field->id_size = reg_field.id_size;
 	rm_field->id_offset = reg_field.id_offset;
 }