net: phy: marvell: fix HWMON enable register for 6390

Register 27_6.15:14 has the following description in 88E6393X
documentation:
  Temperature Sensor Enable
    0x0 - Sample every 1s
    0x1 - Sense rate decided by bits 10:8 of this register
    0x2 - Use 26_6.5 (One shot Temperature Sample) to enable
    0x3 - Disable

This is compatible with how the 6390 code uses this register currently,
but the 6390 code handles it as two 1-bit registers (somewhat), instead
of one register with 4 possible values.

(A newer version of the 6390 documentation removed temperature sensor
 section completely. In an older version, the above mentioned register
 is reserved, although it is R/W. Since the code works, I think we can
 assume that it is correct.)

Rename this register and define all 4 values according to 6393X
documentation.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>