USB: serial: f81232: fix incomplete serial port generation

USB: serial: f81232: fix incomplete serial port generation

The Fintek F81532A/534A/535/536 family relies on the
F81534A_CTRL_CMD_ENABLE_PORT (116h) register during initialization to
both determine serial port status and control port creation. If the
driver experiences fast load/unload cycles, the device state may becomes
unstable, resulting in the incomplete generation of serial ports.

Performing a dummy read operation on the register prior to the initial
write command resolves the issue. This clears the device's stale internal
state. Subsequent write operations will correctly generate all serial
ports.

This patch also removes the retry loop in f81534a_ctrl_set_register()
because the stale state has been fixed.

Tested on: HygonDM1SLT(Hygon C86 3250 8-core Processor)

Signed-off-by: Ji-Ze Hong (Peter Hong) <peter_hong@fintek.com.tw>
Signed-off-by: Johan Hovold <johan@kernel.org>