serial: obmf-icp: avoid UART pin override control

Do not set RX_OVERRIDE_ENABLE or TX_OVERRIDE_ENABLE during normal TTY
RX/TX control operations. These bits request optional GPIO-based pin
override behavior and may be rejected by firmware when the board does not
provide rx-override-gpios or tx-override-gpios.

Clear the override-enable bits when updating SERIAL_CONTROL and only
update the RX_STATE/TX_STATE bits for the normal control path, avoiding
spurious control rejected errors on open and close.

Google-Bug-Id: 568397159
Google-Bug-Id: 554091669
Tested: Built linux-gbmc (gbmc-6.18-ast27xx) and verified on Raya BMC pnhuna-mfq7-bmc-01 (3x Zephyr IOX GPIO chips enumerated)
Change-Id: I31c05bb1d694604697e1c4760cd831c3594060a0
Signed-off-by: Pradeep Nalla <pnalla@marvell.com>