staging: rtl8712: remove unused drvinfo_sz from update_recvframe_attrib

staging: rtl8712: remove unused drvinfo_sz from update_recvframe_attrib

The drvinfo_sz is a size of hardware generated data putting in front of
real RX data. The functions r8712_rxcmd_event_hdl() and recvbuf2recvframe()
have its own parsing code to get drvinfo_sz to access real RX data, so
removing this unused drvinfo_sz is safe.

Otherwise, clang report:

rtl8712_recv.c:139:6: warning:
	variable 'drvinfo_sz' set but not used [-Wunused-but-set-variable]
  139 |         u16 drvinfo_sz;
      |             ^

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/20240913002815.5149-2-pkshih@realtek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>