power: pmic: tps65218: Fix tps65218_voltage_update function

power: pmic: tps65218: Fix tps65218_voltage_update function

Currently while setting the vsel value for dcdc1 and dcdc2
the driver is wrongly masking the entire 8 bits in the process
clearing PFM (bit7) field as well. Hence describe an appropriate
mask for vsel field and modify only those bits in the vsel
mask.

Source: http://www.ti.com/lit/ds/symlink/tps65218.pdf

Signed-off-by: Keerthy <j-keerthy@ti.com>
Fixes: 86db550b38 ("power: Add support for the TPS65218 PMIC")
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>