regulator: pbias: Fix is_enabled callback implementation

The is_enabled implementation is wrong in some cases:
e.g. for pbias_mmc_omap5: enable_mask is : BIT(27) | BIT(25) | BIT(26)
However, pbias_regulator_enable() only sets BIT(27) | BIT(26) bits.
So is_enabled callback will always return false in this case.
Fix the logic to compare the register value with info->enable rather than
info->enable_mask.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>