staging: fbtft: simplify array index computation

staging: fbtft: simplify array index computation

An array index is being computed by mathematical calculation on the
Lvalue side of the expression. This also further results in the statement
exceeding 80 character statement length.

A local variable can store the value of the array index computation. The
variable can then be used as array index. This improves readability of
the code and also address 80 character warning raised by checkpatch.

Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Link: https://lore.kernel.org/r/4bf407e1b3e05745f767b2bad6218c9fb836d869.1584314604.git.mh12gx2825@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>