drivers/pwm: pwm-fsl-ftm: use common header for FlexTimer #defines

This also fixes the wrong value for the previously defined
FTM_MODE_INIT macro (it was not used).

Reviewed-by: Esben Haabendal <esben@haabendal.dk>
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>