mmc: fsl_esdhc: Fix SDR104 and HS200 support

The 3V3/1V8 switching could never have worked on any of the iMXes
ever since 51313b49 ("mmc: fsl_esdhc: support SDR104 and HS200"),
because that commit uses priv->vqmmc_dev when switching voltages on
mode switch, while local vqmmc_dev in probe to store the regulator
pointer. Those are two different variables with the same name. So
the priv->vqmmc_dev was always NULL and thus voltage switch between
modes never really suceeded.

Fix this by assigning priv->vqmmc_dev with value of the vqmmc_dev
in probe.

Fixes: 51313b49 ("mmc: fsl_esdhc: support SDR104 and HS200")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>