firmware: tegra: bpmp: Drop unused mbox_client_to_bpmp()

commit 9c3a62c2 upstream.

mbox_client_to_bpmp() is not used, W=1 builds:

  drivers/firmware/tegra/bpmp.c:28:1: error: unused function 'mbox_client_to_bpmp' [-Werror,-Wunused-function]

Fixes: cdfa358b ("firmware: tegra: Refactor BPMP driver")
Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>