drivers: net: cpsw: fix for cpsw crash when build as modules

When CPSW and Davinci MDIO are build as modules, CPSW crashes when
accessing CPSW registers in CPSW probe. The same is working in built-in
as the CPSW clocks are enabled in Davindi MDIO probe, SO Enabling the
clocks before accessing the version register and moving out the other
register access to cpsw device open.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>