serial: 8250: use hashtable

Instead of open-coding the hash table, use the one provided by
hashtable.h. The semantics is the same, except the code needs not to
compute the hash bucket on its own.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250611100319.186924-33-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>