[comboot] Fix stack pointer retrieval after COM32 binary returns

[comboot] Fix stack pointer retrieval after COM32 binary returns

This change fixes the offset used when retrieving the iPXE stack
pointer after a COM32 binary returns.  The iPXE stack pointer is saved
at the top of the available memory then the the top of the stack for
the COM32 binary is set just below it.  However seven more items are
pushed on the COM32 stack before the entry point is invoked so when
the COM32 binary returns the location of the iPXE stack pointer is 28
(and not 24) bytes above the current stack pointer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>