env: cosmetic: drop assignment i = iomux_doenv()

iomux_doenv() can only return 0 or 1.
So there is no need to save its return value in variable i, as checking
its truth value within an if statement is enough.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Reviewed-by: Marek Vasut <marex@denx.de>