mlxsw: pci: Simplify FRC clock reading

Currently, the reading of FRC values (high and low) is done using macro
which calls to a function. In addition, to calculate the offset of FRC,
a simple macro is used. This code can be simplified by adding an helper
function and calculating the offset explicitly instead of using an
additional macro for that.

Add the helper function and convert the existing code. This helper will be
used later to read UTC clock.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>