drivers: can: sja1000: reduce number of `frame->id` pointer dereferences

Refined `can_sja1000_read_frame` and `can_sja1000_write_frame` by
reducing `frame->id` pointer dereferences.
Using a local `id` variable aims to improve efficiency, given
the frequent execution of this code in CAN applications.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>