mlxsw: pci: Move software reset code to a separate function

In general, the existing flow of software reset in the driver is:
1. Wait for system ready status.
2. Send MRSR command, to start the reset.
3. Wait for system ready status.

This flow will be extended once a new reset command is supported. As a
preparation, move step #2 to a separate function.

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: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>