stlink: remove only instance of useconds_t

stlink: remove only instance of useconds_t

The usleep() function, and its associated useconds_t type
specifier, has been obsoleted by POSIX.1-2008.

OpenOCD has 28 call to usleep(), that should be migrated to the
replacement nanosleep(), but due to the different prototype
	int nanosleep(const struct timespec *req, struct timespec *rem);
this can take some effort.

The type useconds_t is used in only one case, where it's used both
as parameter of usleep() and as value passed to LOG_DEBUG(). Due
to different implementation of useconds_t, there are cases that
trigger a compile warning in LOG_DEBUG() when useconds_t is more
than 32 bit.
E.g. with unistd.h in MinGW 4.x, useconds_t is defined as unsigned
long, thus being 32 or 64 bits depending on the target.

Replace the only instance of useconds_t.

Change-Id: I21724f8b06780abdb003a57222ff1d6840ff5419
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5544
Tested-by: jenkins
Reviewed-by: Ake Rehnman <ake.rehnman@gmail.com>