eth/r8152: modify rtl_clear_bp function

The original rtl_clear_bp() is used to clear the firmware of both
PLA and USB MCU. The new one could clear the firmware of PLA or
USB independently. It is unnecessary to clear firmware, if there
is no one to be updated.

Besides, clear the firmware by writing the relative registers in
bulk.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>