tty: serial: 8250: Fix port count mismatch with the device

Normally, the number of ports is indicated by the third digit of the
device ID on Moxa PCI serial boards. For example, `0x1121` indicates a
device with 2 ports.

However, `CP116E_A_A` and `CP116E_A_B` are exceptions; they have 8
ports, but the third digit of the device ID is `6`.

This patch introduces a function to retrieve the number of ports on Moxa
PCI serial boards, addressing the issue described above.

Fixes: 37058fd5

 ("tty: serial: 8250: Add support for MOXA Mini PCIe boards")
Cc: stable <stable@kernel.org>
Signed-off-by: Crescent Hsieh <crescentcy.hsieh@moxa.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240617063058.18866-1-crescentcy.hsieh@moxa.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>