tg3: Fix crc errors on jumbo frame receive

TG3_PHY_AUXCTL_SMDSP_ENABLE/DISABLE macros do a blind write to the phy
auxiliary control register and overwrite the EXT_PKT_LEN (bit 14) resulting
in intermittent crc errors on jumbo frames with some link partners. Change
the code to do a read/modify/write.

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>