gpio: omap: improve coding style for pin config flags

Change the handling of pin config flags from if/else to switch
statement to make the code more readable and cleaner.

Suggested-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200722120755.230741-1-drew@beagleboard.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>