luci-theme-openwrt: fix scrolling issue in syslog

luci-theme-openwrt: fix scrolling issue in syslog

Depending on the browser, it is sometimes difficult to scroll if moused
over or after clicking or highlighting text. I often have to click outside
the text to be able to scroll again. The browser perceives that there is
scrolling space/content within the <textarea> element because it is
almost, but not exactly, the same size as its parent <div> element,
when in reality, there is no more text content. This causes scrolling
within the element with a range of just a few pixels, and the rest of
the page remains static.

This is easily solved by adding the "overflow-y" property with value
"hidden". Tested by myself on Chrome 64-bit Windows 10

Signed-off-by: Michael Pratt <mpratt51@gmail.com>
[rewrap commit message, fix Sob syntax]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>