drivers: can: mcux: flexcan: rename internal funtions to not use zcan

Rename the NXP FlexCAN driver internal functions and macros to not use
"zcan", "zframe", and "zfilter" to avoid confusion after "struct can_frame"
and "struct can_filter" renaming.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>