davinci: aemif: signedness bug in davinci_aemif_setup_timing()

aemif_calc_rate() can return a negative error value, so all the
variables that get tested for this value need to be signed.

The maximum bit width of WSETUP(WSETUP_MAX) appears to be 30 bits
(0xf << 26). Using a signed instead of an unsigned integer
shouldn't make a difference here.

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>