staging: gdm724x: fix incorrect type in assignment

Fix sparse warning issue.

drivers/staging/gdm724x/gdm_lte.c:201:33: warning: incorrect type in assignment (different base types)
drivers/staging/gdm724x/gdm_lte.c:201:33:    expected unsigned int [unsigned] [addressable] [usertype] ph_len
drivers/staging/gdm724x/gdm_lte.c:201:33:    got restricted __be16 [usertype] payload_len

Signed-off-by: Javier Rodriguez <jrodbar@yahoo.es>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>