ASoC: wm8903: Fix define for WM8903_VMID_RES_250K

VMID Control 0 BIT[2:1] is VMID Divider Enable and Select

00 = VMID disabled (for OFF mode)
01 = 2 x 50kΩ divider (for normal operation)
10 = 2 x 250kΩ divider (for low power standby)
11 = 2 x 5kΩ divider (for fast start-up)

So WM8903_VMID_RES_250K should be 2 << 1, which is 4.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org