bnx2x: fix compilation without CONFIG_BNX2X_SRIOV

Move mutex initialization by allocation of the mailbox it protects.

introduced in commit 1d6f3cd8 'bnx2x: Prevent VF race'

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>