regulator: max8649 - fix setting extclk_freq

The SYNC bits are BIT6 and BIT7 of MAX8649_SYNC register.
pdata->extclk_freq could be [0|1|2].
(MAX8649_EXTCLK_26MHZ|MAX8649_EXTCLK_13MHZ|MAX8649_EXTCLK_19MHZ)
It requires to left shift 6 bits to properly set extclk_freq.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>