i2c: avoid dynamic stack use in dm_i2c_write

The size of the dynamic stack allocation here is bounded by the if()
statement. However, just allocating the maximum size up-front and
doing malloc() if necessary avoids code duplication (the
i2c_setup_offset() until the invocation of ->xfer), and generates much
better (smaller) code:

bloat-o-meter drivers/i2c/i2c-uclass.o.{0,1}
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-144 (-144)
Function                                     old     new   delta
dm_i2c_write                                 552     408    -144
Total: Before=3828, After=3684, chg -3.76%

It also makes static analysis of maximum stack usage (using the .su
files that are automatically generated during build) easier if there
are no lines saying "dynamic".

[This is not entirely equivalent to the existing code; this now uses
the stack for len <= 64 rather than len <= 63, but that seems like a
more natural limit.]

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Heiko Schocher <hs@denx.de>