regulator: max8973: Fix up control flag option for bias control

The control flag for the bias control is MAX8973_CONTROL_BIAS_ENABLE
rather than MAX8973_BIAS_ENABLE which is macro for the bits in
register.

Fix this typo.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>