staging: vchi: change type 'int32_t' to 's32'

As one can see in a typedef at linux/types.h:103, s32 and int32_t are
both a 32 bits signed integer. s32 is preferred because it is defined
by the kernel maintainers and it prevents portability issues. Changed
to meet checkpatch criteria.

Signed-off-by: André Almeida <andrealmeid@riseup.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>