mmc: core: Allow dynamical updates of the number of requests for hsq

mmc: core: Allow dynamical updates of the number of requests for hsq

To allow dynamical updates of the current number of used in-flight
requests, let's move away from using a hard-coded value to a use a
corresponding variable in the struct mmc_host.

This can be valuable when optimizing for certain I/O request sequences, as
shown by subsequent changes.

Signed-off-by: Wenchao Chen <wenchao.chen@unisoc.com>
Link: https://lore.kernel.org/r/20230919074707.25517-2-wenchao.chen@unisoc.com
[Ulf: Re-wrote the commitmsg to clarify the change]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>