net: usb: r8152: Provide missing documentation for some struct members

Fixes the following W=1 kernel build warning(s):

 drivers/net/usb/r8152.c:934: warning: Function parameter or member 'blk_hdr' not described in 'fw_mac'
 drivers/net/usb/r8152.c:934: warning: Function parameter or member 'reserved' not described in 'fw_mac'
 drivers/net/usb/r8152.c:947: warning: Function parameter or member 'blk_hdr' not described in 'fw_phy_patch_key'
 drivers/net/usb/r8152.c:947: warning: Function parameter or member 'reserved' not described in 'fw_phy_patch_key'
 drivers/net/usb/r8152.c:986: warning: Function parameter or member 'blk_hdr' not described in 'fw_phy_nc'
 drivers/net/usb/r8152.c:986: warning: Function parameter or member 'mode_pre' not described in 'fw_phy_nc'
 drivers/net/usb/r8152.c:986: warning: Function parameter or member 'mode_post' not described in 'fw_phy_nc'
 drivers/net/usb/r8152.c:986: warning: Function parameter or member 'reserved' not described in 'fw_phy_nc'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Hayes Wang <hayeswang@realtek.com>
Link: https://lore.kernel.org/r/20201102114512.1062724-23-lee.jones@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>