You want something like this:
<xsl:attribute name="onclick">
<xsl:text>submit(</xsl:text>
<xsl:for-each select="channel">
<xsl:value-of select="@id"/>
<xsl:if test="position() != last()">,</xsl:if>
</xsl:for-each>
<xsl:text>)</xsl:text>
</xsl:attribute>
It's easier in XSLT 2.0:
<xsl:attribute name="onclick">
<xsl:text>submit(</xsl:text>
<xsl:value-of select="channel/@id" separator=","/>
<xsl:text>)</xsl:text>
</xsl:attribute>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference