LinuxKPI: add fsleep()

Add fsleep() function now required by rtw88.  This seems to be
making a decision depending on time to sleep on how to sleep.
Given our compat framework already is lenient on how long to sleep,
this is a cut down version.

MFC after:	10 days
Reviewed by:	hselasky
Differential Revision: https://reviews.freebsd.org/D31322

sys/compat/linuxkpi/common/include/linux/delay.h

@@ -76,4 +76,14 @@ usleep_range(unsigned long min, unsigned long max)
 
 extern unsigned int linux_msleep_interruptible(unsigned int ms);
 
+static inline void
+fsleep(unsigned long us)
+{
+
+	if (us < 10)
+		udelay(us);
+	else
+		usleep_range(us, us);
+}
+
 #endif	/* _LINUX_DELAY_H_ */