i2c: mux: Simplify boolean assignment in i2c_mux_alloc

Refactor boolean field assignments of the form `if (a) b = true` to
`b = !!(a)` in i2c_mux_alloc.

Signed-off-by: I Viswanath <viswanathiyyappan@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>