USB: fix coccinelle warnings

drivers/usb/host/ehci-mv.c:181:26-27: WARNING comparing pointer to 0, suggest !E
/c/kernel-tests/src/cocci/drivers/usb/host/ehci-mv.c:181:26-27: WARNING comparing pointer to 0

 Compare pointer-typed values to NULL rather than 0

Semantic patch information:
 This makes an effort to choose between !x and x == NULL.  !x is used
 if it has previously been used with the function used to initialize x.
 This relies on type information.  More type information can be obtained
 using the option -all_includes and the option -I to specify an
 include path.

Generated by: coccinelle/null/badzero.cocci

CC: Jingoo Han <jg1.han@samsung.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>