The driver may sleep under a spinlock.
The function call path is:
rtc7301_read_time (acquire the spinlock)
  rtc7301_wait_while_busy
    usleep_range --> may sleep

To fix it, usleep_range is replaced with udelay.

This bug is found by my static analysis tool(DSAC) and checked by my code review.
Signed-off-by: NJia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>