staging: rtl8192u: Delete 'fwSendNullPacket' function in r819xU_firmware.c

Delete the fwSendNullPacket function that is not used in anywhere in
the driver.

Fix sparse warnings:
drivers/staging/rtl8192u/r819xU_firmware.c:109:1: warning: symbol 'fwSendNullPacket' was not declared. Should it be static?

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>