[media] em28xx: rename I2C timeout to EM28XX_I2C_XFER_TIMEOUT

This macro is used by all em28xx devices, and not just em2800.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>