spi: mxc_spi: Fix ECSPI reset handling

Reviewing the ECSPI reset handling shows two issues:

1. For the enable/reset bit (MXC_CSPICTRL_EN) in the control reg
   (ECSPIx_CONGREG) the i.MX6 technical reference manual states:

   -- cut --
   ECSPIx_CONREG[0]: EN: Writing zero to this bit disables the block
   and resets the internal logic with the exception of the ECSPI_CONREG.
   -- cut --

   Note the exception mentioned: The CONREG itself isn't reset.

   Fix this by manually writing the reset value 0 to the whole register.
   This sets the EN bit to zero, too (i.e. includes the old
   ~MXC_CSPICTRL_EN).

2. We want to reset the whole SPI block here. So it makes no sense
   to first read the old value of the CONREG and write it back, later.
   This will give us the old (historic/random) value of the CONREG back.
   And doesn't reset the CONREG.

   To get a clean CONREG after the reset of the block, too, don't use
   the old (historic/random) value of the CONREG while doing the reset.
   And read the clean CONREG after the reset.

This was found while working on a SPI boot device where the i.MX6 boot
ROM has already initialized the SPI block. The initialization by the
boot ROM might be different to what the U-Boot driver wants to configure.
I.e. we need a clean reset of SPI block, including the CONREG.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>