usb: ehci-mxc: check for pdata before dereferencing

Commit 7e8d5cd9

 ("USB: Add EHCI support for MX27 and MX31 based
boards") introduced code that could potentially lead to a NULL pointer
dereference on driver removal.

Fix this by checking for the value of pdata before dereferencing it.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org> # 2.6.33+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>