OMAP3+: VC: cleanup i2c slave address configuration

- Add an i2c_slave_address field to the omap_vc_channel
- use VC/VP read/modify/write helper instead of open-coding
- remove smps_sa_shift, use __ffs(mask) for shift value
- I2C addresses 10-bit, change size to u16

Special thanks to Shweta Gulati <shweta.gulati@ti.com> for suggesting
the use of __ffs(x) instead of ffs(x) - 1.

Signed-off-by: Kevin Hilman <khilman@ti.com>