staging: dgnc: Fix a sleep-in-atomic-context bug in cls_assert_modem_signals()

The driver may sleep with holding a spinlock.
The function call path (from bottom to top) in Linux-4.17 is:

[FUNC] usleep_range
drivers/staging/dgnc/dgnc_cls.c, 391:
	usleep_range in cls_assert_modem_signals
drivers/staging/dgnc/dgnc_cls.c, 449:
	cls_assert_modem_signals in cls_copy_data_from_queue_to_uart
drivers/staging/dgnc/dgnc_cls.c, 406:
	_raw_spin_lock_irqsave in cls_copy_data_from_queue_to_uart

To fix this bug, usleep_range() is replaced with udelay().

This bug is found by my static analysis tool DSAC.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>