arm: socfpga: mailbox: Fix off-by-one error on command length checking

A mailbox command contains 1-u32 header + arguments. The "len" variable
only contains the length of the arguments, but not the 1-u32 header.
Include the length of header when checking the ring buffer space to
prevent off-by-one error.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>