usb: fix linux header include setup phase

In usb.inc all required Linux include files of the Linux code are looked up
by 'sed'ing through the files of SRC_C and SRC_CC. The Linux include files
are than added as dependency to the SRC_C and SRC_CC files and during dependency
resolution symbolic links to the lx_emul.h will be created.

In the platform specific usb.mk file there are the Linux driver files
added, but unfortunately after including usb.inc. So, for them no dependency
to any Linux include header file is generated and so no symbolic files are
generated.

If the driver code file is compiled as first, as for asix.c, the symbolic links
of the include files are missing and compilation fails.

Add the Linux driver code files to SRC_C before including usb.inc in the
platform specific usb.mk files.