isdn: capi: fix mismatched prototypes

gcc-11 complains about a prototype declaration that is different
from the function definition:

drivers/isdn/capi/kcapi.c:724:44: error: argument 2 of type ‘u8 *’ {aka ‘unsigned char *’} declared as a pointer [-Werror=array-parameter=]
  724 | u16 capi20_get_manufacturer(u32 contr, u8 *buf)
      |                                        ~~~~^~~
In file included from drivers/isdn/capi/kcapi.c:13:
drivers/isdn/capi/kcapi.h:62:43: note: previously declared as an array ‘u8[64]’ {aka ‘unsigned char[64]’}
   62 | u16 capi20_get_manufacturer(u32 contr, u8 buf[CAPI_MANUFACTURER_LEN]);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/isdn/capi/kcapi.c:790:38: error: argument 2 of type ‘u8 *’ {aka ‘unsigned char *’} declared as a pointer [-Werror=array-parameter=]
  790 | u16 capi20_get_serial(u32 contr, u8 *serial)
      |                                  ~~~~^~~~~~
In file included from drivers/isdn/capi/kcapi.c:13:
drivers/isdn/capi/kcapi.h:64:37: note: previously declared as an array ‘u8[8]’ {aka ‘unsigned char[8]’}
   64 | u16 capi20_get_serial(u32 contr, u8 serial[CAPI_SERIAL_LEN]);
      |                                  ~~~^~~~~~~~~~~~~~~~~~~~~~~

Change the definition to make them match.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>