spi: Trivial warning fix

The loop count i traverses for ntrans which is unsigned
so make the loop count i also unsigned.

Fix the below warning
In file included from drivers/spi/spi-omap2-mcspi.c:38:
include/linux/spi/spi.h: In function 'spi_message_alloc':
include/linux/spi/spi.h:556: warning: comparison between signed and unsigned integer expressions

Cc: Vitaly Wool <vwool@ru.mvista.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

include/linux/spi/spi.h

@@ -600,7 +600,7 @@ static inline struct spi_message *spi_message_alloc(unsigned ntrans, gfp_t flags
 			+ ntrans * sizeof(struct spi_transfer),
 			flags);
 	if (m) {
-		int i;
+		unsigned i;
 		struct spi_transfer *t = (struct spi_transfer *)(m + 1);
 
 		INIT_LIST_HEAD(&m->transfers);