Centralize the switch shutdown routine in a dedicated function,
ksz_switch_shutdown(), to enhance code maintainability and reduce
redundancy. This change abstracts the common shutdown operations
previously duplicated in ksz9477_i2c_shutdown() and ksz_spi_shutdown().
This refactoring is a preparatory step for an upcoming patch to avoid
reset on shutdown if Wake-on-LAN (WoL) is enabled.
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>