C6X: fix KSTK_EIP and KSTK_ESP macros

There was a latent typo in the C6X KSTK_EIP and KSTK_ESP macros which
caused a problem with a new patch which used them. The broken definitions
were of the form:

  #define KSTK_FOO(tsk) (task_pt_regs(task)->foo)

Note the use of task vs tsk. This actually worked before because the
only place in the kernel which used these macros passed in a local
pointer named task.

Signed-off-by: Mark Salter <msalter@redhat.com>