net: phy: mscc: ptp: fix a smatch error

The following error was reported by smatch:
vsc85xx_ts_read_csr() error: uninitialized symbol 'blk_hw'.

In practice this is very unlikely, as all the block identifiers given to
this functions are handled and described in an enum. The smatch error is
fixed by doing what is already done in vsc85xx_ts_write_csr: using the
"PROCESSOR" block by default.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>