Change the function definition to map the declaration and consistently
return an enum value. This fixes the following error reported by GCC:

sys/contrib/vchiq/interface/vchiq_arm/vchiq_2835_arm.c:322:1: error: conflicting types for 'vchiq_copy_from_user' due to enum/integer mismatch; have 'int(void *, const void *, int)' [-Werror=enum-int-mismatch]

322 | vchiq_copy_from_user(void *dst, const void *src, int size)
    | ^~~~~~~~~~~~~~~~~~~~

In file included from sys/contrib/vchiq/interface/vchiq_arm/vchiq_arm.h:38,

from sys/contrib/vchiq/interface/vchiq_arm/vchiq_2835_arm.c:61:

sys/contrib/vchiq/interface/vchiq_arm/vchiq_core.h:647:1: note: previous declaration of 'vchiq_copy_from_user' with type 'VCHIQ_STATUS_T(void *, const void *, int)'

647 | vchiq_copy_from_user(void *dst, const void *src, int size);
    | ^~~~~~~~~~~~~~~~~~~~