staging: sm750fb: Fix of long line in makefile variable

A simple fix of long line in makefile variable assignment.
Found using checkpatch.pl.

Signed-off-by: R Veera Kumar <vkor@vkten.in>
Link: https://lore.kernel.org/r/20200319095024.GA2970@tulip.local


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>