spi: Replace custom fsleep() implementation

_spi_transfer_delay_ns() partially reimplements what fsleep() does.
Replace that code by calling fsleep() instead.

Signed-off-by: Andy Shevchenko <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi.c

···
 	} else {
 		u32 us = DIV_ROUND_UP(ns, NSEC_PER_USEC);
 
-		if (us <= 10)
-			udelay(us);
-		else
-			usleep_range(us, us + DIV_ROUND_UP(us, 10));
+		fsleep(us);
 	}
 }