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