libusb_cpu_to_le16: macro->static inline function

libusb_cpu_to_le16: macro->static inline function

The libusb_cpu_to_le16 macro was a ({ ... }) expression, which
generates the following compiler warning every time it is used,
(and that is several times for every #include <libusb.h>):

libusb.h:880: warning: ISO C forbids braced-groups within expressions

With this patch, #include <libusb.h> stops generating compiler
warnings on gcc 4.4.

As libusb.h heavily relies on the use of static inline functions
those can be relied on to work properly, and there should not be
any significant difference in the code the compiler generates.