i2c: Fix deselection of muxes

Due to an uninitialised variable, when muxes were deselected, any value
could be written to the mux control register. On the PCA9548, this could
result in multiple channels being selected, thus enabling multiple
pull-up resistors, and much bus capacitance.

The fix is simply to initialise the written value to zero.

Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>