serial: arm_dcc: Fix compilation warning and remove unneeded initialization

- arm_dcc_dev is already initialized.
- Remove unused rc variable
Warning log:
arm_dcc.c: In function 'drv_arm_dcc_init':
arm_dcc.c:145:6: warning: unused variable 'rc' [-Wunused-variable]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Marek Vasut <marex@denx.de>