spi: davinci: check for NULL buffer pointer before using it
spi: davinci: check for NULL buffer pointer before using it

In the davinci_spi_{tx|rx}_u{8|16}() functions, check for
buffer pointer being valid before using it. While providing
for better error checking, this change will help simplify
code in the caller.

Signed-off-by: Brian Niebuhr <a49a83b0cf729bda9058567ff35194aeb3c10914@efjohnson.com>
Tested-By: Michael Williamson <06aa4020ece94d3ea8bdaca03e28eaba37f5c2e6@criticallink.com>
Signed-off-by: Sekhar Nori <946cfb81282bb55c59f600e7ee3d3b2c6973f7a3@ti.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs