drivers/spi/spi-generic: Fix typo

'useable' -> 'usable'.

Change-Id: Iad2e1ef97a5c98ffa6f33806bce5b77722e687e8
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77524
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
diff --git a/src/drivers/spi/spi-generic.c b/src/drivers/spi/spi-generic.c
index e6ec7bd..c1ecd67 100644
--- a/src/drivers/spi/spi-generic.c
+++ b/src/drivers/spi/spi-generic.c
@@ -98,7 +98,7 @@
 	if (deduct_opcode_len)
 		cmd_len--;
 
-	/* Subtract command length from useable buffer size. If
+	/* Subtract command length from usable buffer size. If
 	   deduct_opcode_len is set, only subtract the number command bytes
 	   after the opcode. If the adjusted cmd_len is larger than ctrlr_max
 	   return 0 to inidicate an error. */