net: phy: spi_ks8995: clean up ks8995_registers_read/write

The change removes redundant sysfs binary file boundary checks,
since this task is already done on caller side in fs/sysfs/file.c

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>