mmc: sdhci-pxav3: set_uhs_signaling is initialized twice differently

.set_uhs_signaling field is currently initialised twice once to the
arch specific callback pxav3_set_uhs_signaling, and also to the generic
sdhci_set_uhs_signaling callback.

This means that uhs is currently broken for this platform currently, as pxav3
has some special constriants which means it can't use the generic callback.

This happened in
commit 96d7b78c ("mmc: sdhci: convert sdhci_set_uhs_signaling() into a library function")
commit a702c8ab ("mmc: host: split up sdhci-pxa, create sdhci-pxav3.c")'

Fix this and hopefully prevent it happening in the future by ensuring named
initialisers always follow the declaration order in the structure definition.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <stable@vger.kernel.org> # v3.16+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>