mmc: sdhci-cadence: fix PHY write

Accordingly to Cadence documentation, PHY write procedure is:

1. Software sets the PHY Register Address (HRS04[5:0]) and the
   PHY Write Data (HRS04[15:8]) fields.
2. Software sets the PHY Write Transaction Request (HRS04[24]) field to 1.
3. Software waits as the PHY Write Transaction Acknowledge (HRS04[26])
   field is equal to 0.
4. Hardware performs the write transaction to PHY register where
   HRS04[15:8] is a data written to register under HRS04[5:0] address.
5. Hardware sets the PHY Transaction Acknowledge (HRS04[26]) to 1 when
   transaction is completed.
6. Software clears the PHY Write Transaction Request (HRS04[24]) to 1
   after noticing that the PHY Write Transaction Acknowledge (HRS04[26])
   field is equal to 1.
7. Software waits for the PHY Acknowledge Register (HRS04[26]) field is
   equal to 0.

Add missing steps 3 and 7. Lack of these steps causes
integrity errors detested by hardware.

Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
Link: https://lore.kernel.org/r/20200525074053.7309-1-vladimir.kondratiev@intel.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>