drivers: net: tc6: Add read,modify and write operation on control regs

The OA TC6 driver requires some bits manipulations in control registers.
Up till now - it has been implemented as an explicit set of read and write
registers' operations.
One good example would be the oa_tc6_set_protected_ctrl() implementation,
which used such scheme.

This patch brings dedicated function for this operation; oa_tc6_reg_rmw().
The aforementioned oa_tc6_set_protected_ctrl() function now uses it
internally.

Signed-off-by: Lukasz Majewski <lukma@denx.de>