staging: rtl8723bs: reduce stack usage of cfg80211_rtw_scan

staging: rtl8723bs: reduce stack usage of cfg80211_rtw_scan

The build of xtensa allmodconfig gives warning of:
In function 'cfg80211_rtw_scan':
warning: the frame size of 1040 bytes is larger than 1024 bytes

Allocate memory for ssid dynamically to reduce the stack usage, as an
added benifit we can remove the memset by using kzalloc while allocating
memory.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Link: https://lore.kernel.org/r/20191018101854.31876-1-sudipm.mukherjee@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>