staging: rtl8188eu: Fix a possible sleep-in-atomic-context bug in rtw_disassoc_cmd()

The driver may sleep with holding a spinlock.
The function call paths (from bottom to top) in Linux-4.16.7 are:

[FUNC] kzalloc(GFP_KERNEL)
drivers/staging/rtl8188eu/core/rtw_cmd.c, 502:
		kzalloc in rtw_disassoc_cmd
drivers/staging/rtl8188eu/core/rtw_ioctl_set.c, 256:
		rtw_disassoc_cmd in rtw_set_802_11_ssid
drivers/staging/rtl8188eu/core/rtw_ioctl_set.c, 235:
		spin_lock_bh in rtw_set_802_11_ssid

[FUNC] kzalloc(GFP_KERNEL)
drivers/staging/rtl8188eu/core/rtw_cmd.c, 502:
		kzalloc in rtw_disassoc_cmd
drivers/staging/rtl8188eu/core/rtw_ioctl_set.c, 352:
		rtw_disassoc_cmd in rtw_set_802_11_infrastructure_mode
drivers/staging/rtl8188eu/core/rtw_ioctl_set.c, 336:
		spin_lock_bh in rtw_set_802_11_infrastructure_mode

To fix this bug, GFP_KERNEL is replaced with GFP_ATOMIC.

This bug is found by my static analysis tool (DSAC-2) and checked by
my code review.

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