staging: et131x: Refactor et131x_isr() to remove indenting

By negating a 'status' variable check in et131x_isr(), we can remove
the indenting of a large block of code, increasing the readability.

This patch does exactly that.
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>