mmc: core: simplify return code

File contains multiple functions doing variations on the same thing,
sdio_readb(), sdio_writeb()f, sdio_readw(), sdio_writew()
etc. Although the functions have very similar logic the code is laid
out in a variety of ways. This makes it overly complicated to
read. There is a already a nice clean chunk of code, if we use this
format for all instances then we will have cleaned up the code,
reduced the line count and lessened the cognitive load required while
reading. Less lines equals less bugs.

Pick the most simple and clear code flow and change all functions to
be the same.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>