isdn/capi: correct capi20_manufacturer argument type mismatch

Function capi20_manufacturer() is declared with unsigned int cmd
argument but called with unsigned long.
Fix by correcting the function prototype since the actual argument
is part of the user visible API.

Spotted with Coverity.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>