imx: i2c: Zap unnecessary malloc() calls

The malloc() calls are unnecessary, just allocate the stuff on stack.
While at it, reorder the code a little, so that only one variable is
used for the text, use snprintf() instead of sprintf() and use %01d
as a formatting string to avoid any possible overflows.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Sean Cross <xobs@kosagi.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>