mmc: pwrseq: simplify alloc/free hooks

The alloc() and free() hooks required each pwrseq implementation to set
host->pwrseq themselves. This is error-prone and could be done at a
higher level if alloc() was changed to return a pointer to a struct
mmc_pwrseq instead of an error code.

This patch performs this change and moves the burden of maintaining
host->pwrseq from the power sequence hooks to the pwrseq code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>