Bluetooth: hci_raw: Make bt_buf_get_rx args consistent

The hci_raw version of bt_buf_get_rx was expecting an int for timeout.
Let us int32_t instead so we match both the hci_core version and the
type that net_buf_alloc expects.

This addresses a possible build issue if/when int32_t differs from our
default (ie, newlib).

Change-Id: I69374c48da8f2b96fa2bd418ff505fbaacda11f0
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>