mmc: mmc_spi: Add/rename defines for timeouts

Clarify the use of r1b_timeout, by renaming it to MMC_SPI_R1B_TIMEOUT_MS
and by dropping the corresponding confusing comment about it.

Additionally, let's also add a new define, MMC_SPI_INIT_TIMEOUT_MS and use
it during the initialization. Even if these two defines are given the same
value, the split makes it easier to understand them.

Cc: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200414161413.3036-18-ulf.hansson@linaro.org