net: ftgmac100: access hardware register after clock ready

AST2600 MAC register 0x58 is writable only when the MAC clock is
enabled.  Usually, the MAC clock is enabled by the bootloader so
register 0x58 is set normally when the bootloader is involved.  To make
ast2600 ftgmac100 work without the bootloader, postpone the register
write until the clock is ready.

Fixes: 137d23ce

 ("net: ftgmac100: Fix Aspeed ast2600 TX hang issue")
Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>