drivers: wifi/airoc: update airoc_wifi_host_buffer_get

Update airoc_wifi_host_buffer_get function to set final
buff size based on input “size” param.

Update airoc_mgmt_send to use airoc_wifi_host_buffer_get
instead net_buf_alloc_len.

Signed-off-by: Nazar Palamar <nazar.palamar@infineon.com>