drivers: dma_mcux_lpc: fix bug using INPUTMUX

The typical way of calling INPUTMUX_AttachSignal() is to
bracket it around INPUTMUX_Init() and INPUTMUX_Deinit()
calls because we can reduce power consumption by not
keeping the interface powered when not changing INPUTMUX.

This driver was violating that convention, which caused
it to not coexist well with other code that followed the
usage convention because the INPUTMUX might be initialized
or not depending on execution order with the other modules.

Signed-off-by: Mike J. Chen <mjchen@google.com>