mmc: core: Normalize the error handling branch in sd_read_ext_regs()

mmc: core: Normalize the error handling branch in sd_read_ext_regs()

Let's use pr_err() to output the error messages and let's extend a comment
to clarify why returning 0 (success) in one case make sense.

Fixes: c784f92769ae ("mmc: core: Read the SD function extension registers for power management")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[Ulf: Clarified the comment and the commit-msg]
Link: https://lore.kernel.org/r/20221130134920.2109-1-thunder.leizhen@huawei.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>