drivers/usb/host/ehci-mxc.c: Fix GCC 4.6 warning

Fix:
ehci-mxc.c: In function 'ehci_hcd_init':
ehci-mxc.c:113:6: warning: variable 'tmp' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>