net: stmmac: Fix build errors in dwmac-qcom-pps.c

ethqos_unregister_pps_isr() tries to implicitly cast the return
value of free_irq() from a void * to an int, which causes build
errors. The return value of free_irq() is also ignored. Thus,
do not store the return value of free_irq() in
ethqos_unregister_pps_isr(). Also, remove an unused variable in
ethqos_init_pps().

Change-Id: Id0c0bbed980982666c276ecb88d6148c205078f0
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>