i2c: npcm7xx: Fix timeout calculation

timeout_usec value calculation was wrong, the calculated value
was in msec instead of usec.

Fixes: 56a1485b

 ("i2c: npcm7xx: Add Nuvoton NPCM I2C controller driver")
Signed-off-by: Tali Perry <tali.perry1@gmail.com>
Reviewed-by: Avi Fishman <avifishman70@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Alex Qiu <xqiu@google.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>